Sensor devices for structural health monitoring

ABSTRACT

Described herein are wireless interrogation systems and methods that rely on a complementary sensing device and interrogator. The sensing device is disposed to measure a parameter indicative of the health of a structure. A sensor reading from the sensor indicates the level of a parameter being monitored or whether one or more particular physical or chemical events have taken place. Using wireless techniques, the interrogator probes the device to determine its identity and its current sensor reading. This often includes transmission of a wireless signal through portions of the structure. When activated, the device responds with a wireless signal that identifies the device and contains information about the parameter being measured or a particular sensor state corresponding to the parameter. The identity of the device allows it to be distinguished from a number of similar devices. Thus this invention finds particular usefulness in the context of an array of devices that can be probed by a wireless interrogation unit. In one embodiment, the devices are passive and derive power from the interrogation signal.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation-in-part of a U.S. patentapplication entitled “Wireless Event Recording Devices WithIdentification Codes” by David G. Watters et al., filed on Feb. 26, 1999(U.S. application Ser. No. 09/258,073), which is incorporated byreference herein.

BACKGROUND OF THE INVENTION

[0002] The present invention relates generally to sensor technology.More particularly, the present invention relates to sensing devices andsystems used to monitor large structures using wireless communicationtechniques.

[0003] Many large structures are prone to degradation over time. Forexample, a large structure including metal elements, such as an aircraftor concrete structure with metal reinforcement, is often susceptible tometal corrosion. Active monitoring and maintenance of a structure mayalleviate degradation. Conventional sensing techniques frequently do notprovide suitable monitoring solutions for monitoring the health of alarge structure. For a concrete structure with metal reinforcement, themetal is often embedded within the structure and inaccessible usingsensors that employ a wire for external communication. In someapplications, the structure may be so large that hundreds or thousandsof sensors are needed for health observation and information collection.For structures comprising concrete for example, concrete strength andlifetime performance are strongly dependent on curing conditions.Construction personnel would ideally like to monitor the temperature ofdiscreet points within the concrete during the curing process. This isnot permissible according to conventional techniques.

[0004] Inspection of aging bridge decks, as another example, is animportant component of an effective highway maintenance program.Pavement and concrete exposed to heavy traffic and weatheringdeteriorate over time. There are an abundant number of bridges (about600,000) in United States—and California alone has over 12,000 bridges.Each bridge is inspected on a regular basis. Most of the bridges includemetal elements such as tensile bars and metal grids embedded in theconcrete. Chloride from the environment surrounding the bridge diffusesinto the concrete at its surface, and diffuses from the surface into theconcrete depths. The chloride ions may originate from deicing salts incold climates or salt water from seawater spray in coastal zones, orrunoff from nearby (salty) soil embankments, for example. Over time, theamount of chloride in the concrete increases and may reach levels thatsubstantially attack and corrode the metal elements. For a given metal,there is typically a chloride concentration level where corrosioninitiates. Steel rebar for example will corrode in the presence ofchloride ions at known critical levels. The resulting corrosion leads toexpansion of the rebar volume and cracking of the concrete. Monitoringcorrosion of the metal caused by chloride ingress is a major objectiveof regular highway maintenance inspections. Since the metal bars andgrids are usually embedded within the concrete at a particular depth,chloride levels at varying depths of the concrete may be monitored totrack chloride ion ingress and to detect when chloride presence in theconcrete is approaching levels of interest.

[0005] Current inspection techniques include visual observation andmanual extraction of core samples. Highway engineers frequently takecore samples and submit them for laboratory analysis to determinechloride penetration. This process typically involves removing acylindrical plug from the concrete. Highway engineers then send the plugto a lab and wait for results. Laboratory analysis involves slicing thesample into layers, crushing individual layers, dissolving the layersinto solution, followed by a multi-step titration process to determinethe chloride concentration. In addition to performing this analysis,individual samples need to be labeled and tracked to correlate labmeasurements with specific bridges.

[0006] Not only highly time-consuming, costly, and prone to confusion,this method also aggravates the same problem it intends to detect andprevent. Namely, highway engineers refill the hole with a plug. Giventhe inevitable mismatch of plug materials and sizing, chloride ions nowhave an easier route into the concrete depths, These manual techniquesmay also contribute to traffic congestion. Moreover, it typically takesyears for a critical chloride concentration to be reached, so a largenumber of these inefficient manual tests may be required.

[0007] In view of the foregoing, there are desired improved structuresand techniques for monitoring the health of large structures, such asbridge decks.

SUMMARY OF THE INVENTION

[0008] The present invention improves structural health monitoring byenabling wireless interrogation systems and methods that rely on acomplementary sensor device and interrogator. The sensor devicecomprises a sensor that measures a parameter indicative of the health ofthe structure. A sensor reading from the sensor indicates the level of aparameter being monitored or whether one or more particular physical orchemical events have occurred. For example, the device may include anelectrochemical sensor that measures the level of a chemical species inthe structure and an event may be the attainment of a particularconcentration level of the chemical species. Using wireless techniques,the interrogator probes the sensor device to determine its identity anda current sensor reading or state. Often, the sensor device is embeddedin the structure and transmission of a wireless signal occurs through aportion of the structure. When activated, the device responds with awireless signal that identifies the device and contains informationabout the parameter being measured or a particular sensor state. Theidentity of the device allows it to be distinguished from a number ofsimilar devices. Thus this invention finds particular usefulness in thecontext of an array of devices that can be probed by a wirelessinterrogation unit. In one embodiment, the devices are passive andderive power from the interrogation signal.

[0009] In one aspect, the present invention is applied to monitoring thehealth of roadways, bridges or portions thereof such as bridge decks. Adevice comprising a sensor is embedded in the concrete and measureslocal chloride concentration. When polled by a suitable interrogator,the device outputs a signal corresponding to the chloride concentration.For example, the interrogator may pole the device using an RF signal.Using circuitry that quickly responds to the interrogation signal, thepresent invention enables real-time communication with an embeddedsensor. Each device may be uniquely identified with a number or codestored in a microchip, for example. Interrogation of numerous devices inproximity may use anticollision algorithms and RFID technology. Adatabase may be constructed for a sensor array in a roadway or bridge.The database may be used to track polling results over time for a singlebridge—and allow for convenient comparison of the health status fornumerous bridges. Applying interrogation to hundreds and thousands ofbridges provides an automated and simplified tool to help maintenancetechnicians prioritize maintenance schedules for a large number ofbridges. In one embodiment, the devices are passive and derive powerfrom the RF illumination, thereby alleviating the need for battery powerand battery maintenance so that the embedded sensor devices may last aslong as the bridge or bridge deck.

[0010] Real-time communication with an embedded sensor permits theinterrogator to be placed on a truck or moving vehicle, and polling ofnumerous similar devices embedded in multiple locations of a bridge deckto be performed as the truck drives over each device. For roadwaymaintenance programs, vehicular interrogation in this manner offerssimplified and expeditious polling compared to conventional manualtechniques.

[0011] In one embodiment, a hand-held RF interrogator illuminates alocal region of a structure, powering any embedded sensors in the regionand obtaining sensor data from the sensors. Data may be provided by thesensor in various forms. For example, sensors may provide overlimit orthreshold data that indicate when a parameter, such as a concentrationlevel of an aggressive chemical or species in the structure, has reacheda level of interest. Some parameters measured by sensor devices of thepresent invention include concentration levels, pH, conductivity, epoxymoisture ingress, corrosion of a surrogate, and polarization resistance,for example. By combining sensor feedback with known sensor deviceposition in the structure and sensor data history, changes in theparameter may be tracked over time. Information collection in thismanner applied over an entire structure allows a profile of healthprogression in the entire structure, e.g., the profile of chemicalingress in a bridge deck.

[0012] To keep the sensor device small and simple, it may bepassive—that is, it does not require a self-contained continual powersource for operation (such as a battery). This also extends devicelongevity. Thus, components included in the device such as a sensor andtransponder may be passive. In one example described, a radio frequencyinterrogation signal may provide the transponder power.

[0013] In one aspect, the present invention relates to a devicecomprising a sensor that detects a parameter indicative of the health ofa structure comprising a metal. The device also comprises a transponderin electrical communication with the sensor and that transmits awireless signal through a portion of the structure indicating theparameter status when triggered by a wireless interrogation signal. Thedevice further comprises an identification source in electricalcommunication with the transponder that uniquely identifies the device.

[0014] In another aspect, the present invention relates to a devicedisposed in a structure comprising concrete. The device comprises asensor embedded in the concrete that detects a parameter. The devicealso comprises a transponder in electrical communication with the sensorand that transmits a wireless signal through a portion of the concreteindicating the parameter status. The device further comprises anidentification source in electrical communication with the transponderthat uniquely identifies the device.

[0015] In yet another aspect, the present invention relates to a devicecomprising an electrochemical cell that measures the potentialdifference between a reference electrode and an ion selective electrode.The device also comprises an identification source that uniquelyidentifies the device. The device further comprises a transponder inelectrical communication with the electrochemical cell and in electricalcommunication with the identification source that transmits a wirelesssignal indicating the potential difference and information from theidentification source.

[0016] In still another aspect, the present invention relates to adevice for monitoring the health of a bridge comprising concrete and ametal. The device comprises a sensor, at least partially exposed to theconcrete, that detects chloride presence in the concrete. The devicealso comprises a transponder in electrical communication with the sensorand that transmits a wireless signal through the concrete indicating thelevel of chloride when triggered by a wireless interrogation signal. Thedevice further comprises an identification source in electricalcommunication with the transponder that uniquely identifies the device.The device is passive.

[0017] In another aspect, the present invention relates to a system forreporting the health of a bridge comprising concrete and a metal. Thesystem comprises an array of devices, each of which is embedded in thebridge. Each device has a sensor that detects a parameter indicative ofthe health of the bridge, a transponder in electrical communication withthe sensor and that transmits a wireless signal through a portion of theconcrete indicating the parameter status when triggered by a wirelessinterrogation signal, and an identification source in electricalcommunication with the transponder that uniquely identifies each devicefrom the other devices. The system also comprises an interrogator forexternally probing a device in the array to determine the parameterstatus. The interrogator is designed or configured to read the parameterstatus by (i) providing the wireless interrogation signal to thetransponder and (ii) receiving a wireless response from the device.

[0018] In another aspect, the present invention relates to a method formonitoring the health of a structure comprising concrete and a metal.The method comprises embedding a sensor device in the concrete. Thesensor device comprises a sensor that detects a parameter indicative ofthe health of the structure, an identification source that candistinguish the device from the other similar devices, and atransponder. The method also comprises detecting a parameter statususing the sensor. The method further comprises probing the device withan interrogator that produces a wireless signal that transmits through aportion of the concrete. The method additionally comprises returning awireless signal from the device through a portion of the concrete. Thereturn wireless signal indicates the parameter status. In oneembodiment, the structure is a bridge or a portion of a bridge.

[0019] The present invention finds use in a wide range of applications.The use of an identification code with each recording device allows formonitoring large structures having many spatially separated points thatare to be individually monitored.

[0020] These and other features and advantages of the present inventionwill be described in the following description of the invention andassociated figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1A illustrates a structure portion comprising concrete andmetal and an array of sensing devices of the present invention.

[0022]FIG. 1B illustrates a cross-section of roadway including an arrayof sensing devices in accordance with one embodiment of the presentinvention.

[0023]FIG. 2A is an illustrative representation of a sensor system formonitoring a bridge deck cross-section in accordance with one embodimentof the present invention.

[0024]FIG. 2B illustrates a sensor system comprising an interrogatorcarried by a moving vehicle in accordance with another embodiment of thepresent invention.

[0025]FIG. 3A illustrates a simplified sensor device system inaccordance with one embodiment of the present invention.

[0026]FIG. 3B illustrates an electrical equivalent circuit for thesystem schematized in FIG. 3A in accordance with a specific embodimentof the present invention.

[0027]FIG. 3C illustrates typical components of a commerciallow-frequency RFID sensor device in accordance with another embodimentof the present invention.

[0028]FIG. 3D illustrates a sensor device that communicates informationrelated to a parameter being monitored based on a frequency shift inaccordance with one embodiment of the present invention.

[0029]FIG. 3E illustrates a sensor device that communicates informationrelated to a parameter being monitored based on bit stream inversion inaccordance with one embodiment of the present invention.

[0030]FIG. 3F illustrates a simplified cross-section view of a sensordevice in accordance with another embodiment of the present invention.

[0031]FIG. 4A illustrates a simplified cross-section view of a chlorideion sensor device in accordance with one embodiment of the presentinvention.

[0032]FIG. 4B shows an exemplary circuit diagram corresponding to thechloride ion sensor device of FIG. 4A.

[0033]FIG. 4C illustrates an electrochemical cell sensor in accordancewith one embodiment of the present invention.

[0034]FIG. 4D illustrates a representative potentiometric thresholdmeasurement circuit for the electrochemical cell sensor device of FIG.4C in accordance with one embodiment of the present invention.

[0035]FIG. 4E illustrates a detailed arrangement for a sensor device inaccordance with one embodiment of the present invention.

[0036]FIG. 5 illustrates an organization of sensor parameters useful formonitoring health of a structure in accordance with various embodimentsof the present invention.

[0037]FIG. 6A illustrates a representative circuit for a sensor devicethat detects conductivity in accordance with one embodiment of thepresent invention.

[0038]FIG. 6B illustrates a representative circuit for a sensor devicethat detects pH in accordance with another embodiment of the presentinvention.

[0039]FIG. 6C illustrates a representative circuit for a sensor devicethat detects epoxy moisture ingress in accordance with one embodiment ofthe present invention.

[0040]FIG. 6D illustrates a representative circuit for a sensor devicethat detects multiple corrosion events in accordance with anotherembodiment of the present invention.

[0041]FIG. 7 illustrates an exemplary reader block diagram correspondingto an interrogator in accordance with one embodiment of the presentinvention.

[0042]FIG. 8 is a process flow diagram depicting a typical procedure forusing sensor devices and interrogators of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] The present invention will now be described in detail withreference to a few preferred embodiments thereof as illustrated in theaccompanying drawings. In the following description, numerous specificdetails are set forth in order to provide a thorough understanding ofthe present invention. It will be apparent, however, to one skilled inthe art, that the present invention may be practiced without some or allof these specific details. In other instances, well known process stepsand/or structures have not been described in detail in order to notunnecessarily obscure the present invention.

1. Structural Health Monitoring

[0044] The present invention is well suited to monitor the health of alarge structure or portion thereof. FIG. 1A illustrates a structureportion 10 may be included as part of a 10 building, bridge, road, etcand comprises concrete 11 and metal 12. An array of sensing devices 50are embedded in structure portion 10 and detect a parameter indicativeof the health of structure portion 10. Sensor devices 50 detect andreport physical or chemical parameters, events or states where multiplepoints of interest are to be monitored. Each such point is associatedwith a separate sensing device of this invention. Together the devices50 form a sensor array. Preferably, each sensor device of such array hasa unique identifier that uniquely identifies the device from othersimilar devices and permits the device to provide a suitabledistinguishable reading when probed.

[0045] The wireless monitoring techniques of the present invention aresuitable for health monitoring of various structures. As the term isused herein, a structure generally refers to anything that has beenconstructed. Exemplary structures that are monitored according tovarious embodiments of the present invention include roads, bridges,buildings, railroad tracks, aircraft, pipelines, tunnels, spacecraft,storage tanks, nuclear power plants, and theme-park rides. The buildingsmay include structures such as parking garages, office buildings, seawalls, etc. Railroad tracks may comprise metal and wood designs or metaland concrete designs, for example. Structure as defined herein alsoincludes structural portions and components of a structure such asindividual walls and layers, road and bridge components such as a bridgedeck etc. The structures may comprise any suitable materials such asconcrete, , wood, cement or mortar, asphalt or asphalt concrete,structural honeycomb, glues, liquids (stationary or flowing), plastics,soils, or inaccessible compartments of complex structures such asaircraft or trains, etc. A sensor device of the present invention may bevariably located, attached to, or embedded in any of these non-metalmaterials. As the term is used herein, concrete is intended to refer toany material comprising a conglomerate gravel, broken stone, or slag ina mortar or cement matrix. Two common conventional forms of concreteused for roadways are Portland cement concrete and asphalt concrete.Devices may also be embedded into a variety of non-metallic materialssuch as dielectric, lossy dielectric, or even metal coated materials (solong as the thickness of the metal coat is much less than the skin depthof the material at the wavelength of the interrogation). In addition tothe above-mentioned applications, sensor devices of the presentinvention are generally applicable to any application where rapid orremote inspection of large structures is useful.

[0046] Generally, sensor devices of this invention detect a physical“parameter”. The parameter is usually a physical or chemical property ofan item such as its temperature, density, strain, deformation,acceleration, pressure, mass, opacity, concentration, chemical state,hardness, conductivity, resistance, magnetization, dielectric constant,size, etc. The parameter is typically indicative of the health of thestructure, or a portion thereof. For example, stress or strain invarious portions of the structure can be used to indicate structuralhealth. Alternatively, the parameter or physical property may relate tosome form of structural degradation or threat, such as the presence of achemical that attacks the structure or an element included therein. Theitem whose parameter is being monitored may be included in the structure(e.g., detecting resistance and corrosion of a metal element included ina bridge deck) or added to the structure (e.g., detecting resistance andcorrosion of a surrogate metal included in a sensor device thatphysically or chemically mimics a metal element included in a bridgedeck). For monitoring the health of a metal used in a bridge or bridgeportion for example, the parameter may correspond to chemicalconcentration, pH, conductivity, corrosion levels, and polarizationresistance. In this case, the item being monitored may correspond to ametal element in the bridge or a surrogate item added to the bridge formonitoring purposes.

[0047] Health of a structure generally refers to any condition orphysical parameter pertinent to the functionality of the structure. Thisincludes the health of portions and components included in the structuresuch as separately constructed portions, individual materials,reinforcement beams, cables and bars that contribute to structuralintegrity, adhesives, seals, joints, fasteners, etc. In many cases, afailure mode of the structure is linked to a parameter being sensed by asensor of the present invention. Pavement and concrete exposed toweathering and heavy traffic degrades with time. Pavement failure modesinclude overstress, over strain, temperature induced curling anddeflection, and corrosive attack on metal components within the roadsand bridges such as metal bars and metal grades, for example. A sensordevice may be embedded in concrete to detect each of these conditions.In the last case, the present invention may detect the levels ofaggressive species responsible for corrosion of the metal components.

[0048] In some cases, a sensor device detects an “event” associated witha parameter. The physical or chemical event may correspond to attainmentof a particular value of the physical property. For roadway inspectionapplications, the event may correspond to reaching a particularthreshold concentration level within the concrete for a chemicalaggressive to metal embedded in the concrete. Alternately, a physicalevent may correspond to reaching a particular threshold temperature orhumidity level within concrete during curing. Another physical event maybe a change of state such as a phase change in the item being sensed.

[0049] Examples of phase changes include transformations between gas,liquid, and solid states, changes in morphology (e.g., crystallinestate), magnetization, and the like.

[0050] In one embodiment, sensing devices of the present invention maybe employed to measure overstress thresholds in concrete using apiezoelectric sensor device that is powered by an interrogating RFillumination. These pressure sensitive devices may be manually insertedat the asphalt concrete subgrade interface, at the base of the subgrade,or in load transfer dowels as a part of a dowel retrofit program (seeFIG. 2C). At convenient and subsequent times, a wireless interrogatorilluminates regions within the concrete and pavement, power the embeddedwireless sensor devices, and obtain the tensile and compressive stresseddata. For an array of sensing devices embedded in the concrete, eachdevice may return its own identification code and the desired parameterbeing sensed. For example, each device may return a signal thatindicates whether or not stress in the concrete has exceeded some overlimit condition (such as 400 psi tensile stress in concrete). If so,further validation tests on the concrete may be performed.

[0051] The present invention is also suitable for detecting multipleparameters and events. The multiple parameters and events may beassociated with multiple parameters, multiple thresholds for the sameparameter, or a combination thereof. For example, the physical orchemical events may be different threshold concentration levels ofchloride in concrete 11 of FIG. 1A.

[0052] Note that the parameter or event to be detected could be a veryfast or instantaneous event or one that requires a significant time tounfold. An example of this latter case is reaching a particular chemicalconcentration level in concrete, which may take years. A time-integratedexposure by the event-recording device may be appropriate for long termmonitoring. In this manner, detection of a new steady state conditionalong a continuum of values can be detected and reported.

[0053] Wireless sensing devices of the present invention are well-suitedfor embedding within concrete to monitor the health of structurescomprising concrete. In one embodiment, the devices monitor a parameterduring curing of the concrete, such as temperature, pressure, orhumidity. Concrete strength and lifetime performance are highlydependent on curing conditions. This is especially true forhigh-performance concrete. Knowledge of curing conditions enablesconstruction personnel to estimate lifecycle cost, plan maintenanceactions, and perform quality control on new construction. In a specificembodiment, a sensor device 50 embedded in concrete obtains localtemperature measurements at periodic intervals during curing. Whenpolled by an interrogator of the present invention, the devices respondwith (or without) temperature data that they are configured to detect.Sensors disposed to detect humidity may monitor a hydration process ofcuring concrete. Low-cost wireless temperature sensors of the presentinvention may be widely distributed in a new construction, enabling athorough evaluation of cure integrity. In some cases, these devices mayinclude a small battery that provides DC power and may be polled by anRF interrogator periodically for 28 days after the pour. As a result ofthe measured data, construction personnel may verify cure integrity in astructure and identify spatially distribute regions that may requiremore careful inspection, or rework.

[0054] As mentioned before, there is typically a critical chlorideconcentration level where corrosion initiates for a given metal.Correspondingly, a sensor device 50 of the present invention may beembedded in a structure comprising concrete and a metal (e.g.,reinforced concrete) to detect a chloride concentration threshold level,and/or one or more intermediate concentration threshold levels usefulfor tracking chloride build up to the critical chloride concentrationlevel. Thus, multiple sensor devices 50 may be embedded within structure10 to determine the amount and profile of chloride penetration instructure 10.

[0055] Other applications monitor corrosion detection (in bridges andaircraft for example) by detecting a threshold change in conductivity(e.g., corrosion of a material of interest breaks a circuit connection);water absorption detection (by a hydroscopic material for example) bydetecting a change in EMF, conductivity, ion detection (by precipitationof an insoluble species such AgCl for chloride detection) by detecting achange in opacity, for example. Note that for many of theseapplications, the parameter being detected involves a level of exposure.The device reports how much exposure has occurred. The value ofconductivity, opacity, absorption, etc., correlates to the level ofexposure and the sensor device can report this level.

2. Road and Bridge Health Monitoring Systems

[0056] In particular, the present invention is suitable for periodichealth monitoring of roads and bridges. Inspection of aging pavement andconcrete is an important component of most highway maintenance programs.Although the present invention will now be described primarily withrespect to monitoring the health of bridge decks and concrete basedstructures, the present invention is not limited to these structures andmaterials and is not intended to be limited by examples provided in theexpansion of this embodiment.

[0057]FIG. 1B illustrates a cross-section of roadway 20 including anarray of sensing devices 50 in accordance with one embodiment of thepresent invention. Roadway 20 includes a dual layer constructioncomprising Portland cement concrete 22 disposed below asphalt concrete24. Devices 50 are embedded at various locations within concrete 22 andconcrete 24. For example, devices 50 a and 50 b are laterally disposedto align with wheel paths 26 and monitor degradation of roadway 20 inthese vertical regions. Device 50 c is disposed in the lateralmidsection of roadway 20 and close to the surface 27. Devices 50 a-50 eare disposed at varying depths of roadway 20. Devices 50 a and 50 b areembedded at the upper and lower surfaces of the interface betweenPortland cement concrete 22 and asphalt concrete 24, respectively.Devices 50 c and 50 d are embedded deep within Portland cement concrete22.

[0058] FIGS. 2A-2B illustrate a wireless inspection system formonitoring the health of road structures in accordance with embodimentsof the present invention. FIG. 2A is an illustrative representation of asensor system 30 for monitoring a bridge deck 35 cross-section inaccordance with one embodiment of the present invention. Rebars 36 aredisposed in concrete 34 included in bridge deck 35. Chloride penetrationfrom the surface 38 of bridge deck 35 may contribute to metal corrosionof rebars 36.

[0059] Sensor devices 50 are embedded within bridge deck 30. As shown,devices 50 are embedded within bridge deck 30 using a back-filled core37. In a specific embodiment, each device has an operational depth at aone-inch increment below surface 38. More specifically, device 50 a hasan operational depth one inch below the surface of bridge deck 35,device 50 b has an operational depth two inches below the surface ofbridge deck 35, device 50 c has an operational depth three inches belowthe surface of bridge deck 35, etc. The operational depth refers to thefact that device 50 may be directionally dependent and have a particularportion responsible for sensing and interface with the surroundingstructure. In one embodiment, device 50 includes a port that limitssensor detection to a specific portion of device 50. This allowsdetection of a precise spatial position determined by the precisepositioning and orientation of the port. For example, sensor device 50as shown in FIG. 4A includes a chloride port that allows diffusion ofchloride ions into the sensor at a particular location. For FIG. 2A,each sensor device 50 is disposed with its chloride port facing down andthus the operational depth of each sensor device is measured at thebottom of the sensor device.

[0060] System 30 relies on a hand-held or portable interrogator 32carried by a person. Using wireless techniques, interrogator 32communicates with sensing devices 50 a-50 d. Interrogator 32 produces aprobing signal that penetrates portions of concrete 34 between each ofthe devices 50 and the current position of interrogator 32. In responseto the probing signal from interrogator 32, each device 50 makes asensor reading. Circuitry within device may convert the sensormeasurement into a signal output by a transponder in the device. Thetransponder then returns a response signal to interrogator 32 thatincludes the device's ID and the sensor reading. Interrogator 32 thusallows a person to poll the embedded devices 50 and obtain chlorideingress data in a convenient manner without extracting sensor devices50.

[0061] Devices 50 may be embedded at strategic positions within bridgedeck 30 to detect and report local conditions. This implies thatinterrogator signals to communicate with the embedded device penetratethrough the structural materials. For example, the interrogator may bedesigned to generate and receive RF signals that transmit throughconcrete. When monitoring is performed during curing of the concrete,the RF signals may also penetrate through protective plastic and plywoodcoverings that are used to stabilize curing conditions.

[0062] Thousands of devices 50 may be embedded in a single structure.For example, thousands of devices 50 may be embedded at varyingpositions of interest for a large bridge deck. The devices may beembedded in existing structures or new structures. For FIG. 2A, eachdevice 50 is inserted into a back-filled non-concrete core forevaluation of an existing bridge. For a new bridge, device 50 may beembedded in the concrete as part of the initial concrete pour.

[0063]FIG. 2B illustrates a sensor system 70 in accordance with anotherembodiment of the present invention. As shown, a number of sensors 50and dowels 74 are embedded within roadway 71. Roadway 71 is a two layercomposite comprising asphalt concrete 78 disposed over Portland cementconcrete 79.

[0064] Interrogator 75 is carried by a vehicle 72, such as a highwaymaintenance car, truck or any other suitable moving vehicle.Interrogator 75 includes an antenna disposed below the midsection ofvehicle 72 that allows wireless communication with the sensors 50embedded within roadway 71. Radio frequency power, transmitted on an RFwave from interrogator 75, is used to power wireless transponders ineach sensor device 50. The radio frequency power may also be used topower a sensor and sensor reading, as well as logic in the deviceincluded in a processor or microchip. Each device 50 may then beindividually powered and interrogated as vehicle 72 passes. Logic ineach device may then tell the sensor device to return its identificationcode and a sensor reading to interrogator 75.

[0065] Typically, interrogation using interrogator 75 and vehicle 72comprises driving the truck over roadway 71 at a suitable speed. Theupper speed of vehicle 72 may depend on processing delays to send andreceive a signal from a particular device 50. These delays may include adelay for a probing signal to reach a particular device 50, a delay forthe device 50 to generate a response, a delay for the response signal toreach interrogator 75, and any other processing or wireless transmissiondelays. Regardless of the speed of vehicle 72, inspection of devices 50using system 70 allows convenient (to highway personnel sitting in thevehicle) and less-intrusive (to traffic) methods of road healthinspection.

[0066] In another embodiment, vehicle 72 includes a two-antenna systemcomprising a transmitting antenna at the front of the vehicle andreceiving antenna disposed at the rear. The two antenna system allows awireless device within roadway 71 to receive a probing signal from thefront transmitting antenna, generate a response, and transmit a responsesignal back to rear receiving antenna. This two-antenna system allowsfor increased speed of vehicle 72 since delays in communicating with adevice 50 are compensated by the distance between the front and rearantennas.

[0067] Advantageously, system 70 reduces the time and costs of roadwayinspections such as bridge deck inspections. In some cases, sensordevices 50 of the present invention respond to an interrogator in therange of tens of milliseconds. Regardless of the response time of anembedded sensor device 50, the ability to probe the devices with a truckor other moving vehicle significantly reduces time and costs of bridgedeck inspections for chloride ingress.

3. Principles of Operation

[0068]FIG. 3A illustrates a simplified sensor device system 100 inaccordance with one embodiment of the present invention. An interrogator102 probes sensor devices 104 a and 104 b using wireless communicationand may include any circuitry capable for performing this function. Inthis case, interrogator 102 includes a computer 106, a transceiver 107and an interrogator antenna 108. In one embodiment, coupling betweeninterrogator 102 and the individual event-recording device 104 is byradio frequency (RF) radiation.

[0069] Sensor device 104 a includes rectifier 109, modulator 110, memory112 a including an identification (ID), logic 114, sensor 116, andantenna 117 (which together with modulator 110 serves as a wirelesstransponder).

[0070] Sensor 116 detects a parameter indicative of the health of astructure that device 104 a is disposed or embedded in. Sensor 116broadly refers to any sensor capable of detecting the intended physicalor chemical parameter or event. Numerous sensors are described infurther detail below. Sensor 116 outputs information corresponding tothe current parameter status.

[0071] Sensor 116 is preferably, but not necessarily, a passive sensorthat does not require continual power. In some cases, sensor 116 may beviewed as being energized by the quantity being sensed. For example, ionconcentration gradients may provide the energy to measure a voltagedifference between two electrodes in an electrochemical cell.Alternatively, sensor 116 may be a wire that corrodes in the presence ofa foreign chemical as described with respect to FIG. 6D.

[0072] When probed by interrogator 102, sensor device 104 replies withits ID as stored in memory 112 and information provided by sensor 116 orinformation from a recording mechanism that records an event. Theinformation from sensor 116 indicates the level or status of a parameterbeing monitored. This information is read out along with the device'sidentification code. The ID code provides a mechanism for a) identifyingeach device 104 in a group of devices and b) automatically logging thedata entry corresponding to the sensor reading of each device 104.

[0073] Logic 114 provides instructions for responding to aninterrogation signal by interrogator 102 and includes circuitry andother facilities for preparing a signal to be returned from sensordevice 104 a. This may include processing or altering the output ofsensor 116 for example. Logic 114 may be included in a commerciallyavailable microprocessor, logic device, microchip, etc.

[0074] In some embodiments, the interrogator provides power to theevent-recording device and is transmitted by RF waves, for example.Rectifier 109 of sensor device 104 a rectifies the signal, therebyproviding DC voltage to operate components of device 104 a. In oneembodiment, logic 114 and rectifier 109 are included in the samemicroprocessor.

[0075] Sensor devices of the present invention typically comprise someform of wireless transponder for wireless communication. Generally, thetransponder functions to receive and transmit wireless signals. In somecases, it automatically transmits signals when actuated or probed by asignal from an interrogator. Commonly, a transponder includes anamplifier for increasing the strength of a received incident signal(from the interrogator 102 or other actuating device), a modulator formodifying that signal with information provided to the transponder, andan antenna or antennas for receiving and transmitting. The modulator isthat part of the transponder that impresses information on thetransmitted signal. A “transceiver” may be a component of a transponderresponsible for transmitting and receiving signals, usually independentof one another.

[0076] Note that in the example of FIG. 3A, rectifier 109, modulator 110and antenna 117, together act as a transponder. Rectifier 109 andmodulator 110 communicate with interrogator 102 through antenna 117 andcontain circuitry capable of carrying out this function. This design isspecific to systems employing electromagnetic radiation of anappropriate frequency (e.g., microwave or RF) as the wireless carrier.Other transponder designs are appropriate for other wireless carriersand signals. For example, transponders may be designed for use withacoustic, optical, IR, or electromagnetic sources that are inductivelyor capacitively coupled. Note that the interrogator (or other probingdevice) may employ a multi-band or multi-frequency source having onefrequency to supply power and a second frequency for interrogation, forexample.

[0077] The transponder receives a wireless probe signal from aninterrogator and that signal may include sufficient power to allow itstransmission of the device's identifier and sensor reading back to theinterrogator. The transponder is coupled to the identification memoryand sensor in a manner giving it access to the identification and sensorreading during probing.

[0078] The system 100 of FIG. 3A assumes that the wireless communicationtakes place via electromagnetic radiation of appropriate frequency.Thus, an antenna is used. Generally, however, the interrogator andrecording device may be designed to allow any suitable probe signal orcarrier (not just RF or other electromagnetic radiation). The carriershould allow the device to be probed from a substantial distance andover a wide area. This may include penetration of the signal throughportions of a structure. In some cases, it should also be able to powerthe transmission of data from the sensing device to the interrogator.The carrier should also provide sufficient bandwidth to transfer thedesired information in a timely manner. Additionally, the modulatedcarrier may also be sufficiently unique, in terms of frequency or timesynchronization, or coding, such that it is distinguishable from thesignal provided by nearby sensor devices. Generally, the carrier may bea wave or field or other intangible effector that acts over a distancethrough a medium (vacuum, gas, fluid, solid, etc.) between theinterrogator and the sensor device. Examples of suitable carriersinclude RF radiation, microwave radiation, visible, ultraviolet, andinfrared radiation, acoustic waves, electric fields, magnetic fields,and the like. If the system employs RF radiation, the frequencypreferably ranges between 100 kHz and 5800 MHz and is provided at apower of a few Watts. In a specific embodiment, the interrogator mayoperate at an approved frequency at or near that used for an availableRFID device; e.g., near 125 kHz in one case and about 13 MHz in anothercase. Microwave radiation provides another preferred carrier. Generally,it provides the same functionality as RF radiation, but at larger readranges. Typically, any approved or regulated band such as the ISM bandsat 945 MHz, 5.8 GHz and 2.45 GHz may be used.

[0079] In one embodiment, sensor device 104 a includes a memorycomponent that allows for recording one or more events detected by thesensor 116. The event information is then subsequently provided tointerrogator 102 via the wireless response. In one embodiment of thepresent invention, sensors and memory components capable of recording anevent are incorporated in a microchip, either externally or internally,and may act to change the coded baseband signal directly. Thisadvantageously allows the state change is expressed as a unique code(rather than a subtle change) that can be easily read by theinterrogator.

[0080] In a simple form, the physical or chemical event is recorded bychanging a “1” to a “0” or vice versa. The information recorded (whethera single bit, multiple bits, or some other information) when the eventoccurs can be used in two ways. First, it can be used “directly” byappending to the RFID code such that the reader obtains ID data followedby sensor data. Second, it can be used “indirectly” by selecting one oftwo codes; that is a particular recording device #137 could respond withcode #137 a if the device is normal and code #137 b if the deviceexperienced the event under consideration. From a communications theorypoint of view, these two codes may be orthogonal or nearly orthogonal sothat the reader has a very high probability of distinguishing between anormal device and a transformed device.

[0081] In another embodiment, a silicon-based microelectromechanicalsystem (MEMS) may be employed. Such MEMS are seeing increased usage assensor and actuator systems in a variety of industries. MEMS are smalldevices integrated onto a microchip that may serve as pressure sensors,accelerometers, strain gauges, electrostatic actuators, microswitches,torsional mirrors, etc. These functions result from various MEMsstructures and properties such as capacitance, temperature-dependentsemiconductor activity, electrostatics, Hall effect, magnetostriction,piezoelectric effects, piezoresistance effects, etc. For example, apressure sensor can be implemented in a MEM device in conjunction with aswitch. At the over-limit condition, the deflection of a membrane couldbe used to close a circuit, thus discharging energy or recording theevent. Exemplary MEMs temperature sensors include infrared detectors andthermocouples.

[0082] The memory component may be “unidirectional” with respect to theone or more events such that once an event has occurred, and the memorycomponent has been altered, regressions of the event are unable torevert the memory component to a state used before the event. Forexample, if the event is surpassing a resistance threshold of ametllalic item being monitored, and the memory component converts from afirst state to a second state as a result of surpassing the resistancethreshold, resistances dropping below the resistance threshold will notrevert the memory component to the first state. In many cases, thesensor and parameter being sensed are unable to drive the memorycomponent back to a state used before the event.

[0083] Note that a single structure or mechanism can serve as two ormore of components in sensor device 104. For example, a resonantelectrical circuit can serve as both modulator 110 and antenna 117.Further, a single circuit can serve as both the memory described aboveand some or all of the transponder. For example, some sensor devices usebackscatter modulation to respond to the interrogator. One way toaccomplish this backscatter modulation is to vary the load impedance ofa resonant circuit. The circuit that performs this function (of varyingthe impedance) may be described as modulator and the memory componentthat allows for recording one or more events.

[0084] In one embodiment of operation of system 100, transceiver 107illuminates sensor devices 104 with a short RF pulse. For sensor device104 a, antenna 117 receives the RF signal and rectifies the signal toobtain DC power using rectifier 109. The rectified power is used topower sensor 116, which makes a measurement of the current parameterstatus or status. Logic 114 also receives power for rectifier 109, readsmemory for the sensor device ID 112 a, reads any sensor 116 data orreads data in a memory component used to store an event, and provides adigital signal to modulator 110. Modulator 110 modulates antenna 117backscatter in response to the interrogation signal from interrogator102. The antenna 108 and RF transceiver 107 of interrogator 102 obtainsthe response signal from sensor device 104 a. The response signalincludes ID and sensor data from sensor device 104 a, and reports the IDand sensor data to computer 106. Computer 106 associates with a databaseand updates information for sensor 104 in the database. In some cases,software within computer 106 notes registered bridge health and signalsthe need for further inspection, if necessary.

[0085] Sensor devices of this invention typically include anidentification (ID). Generally, a wireless probe of the sensor deviceshould return a value or other indicator provided by the ID. That valuepreferably uniquely identifies the particular device providing theresponse. This allows it to be distinguished from a number or otherdevices as would be encountered in an array of devices on a system. Inone embodiment, identifier tags are employed. The identifier tags aresmall devices that contain an identification code that can be readremotely using the interrogator. In the case of an array of sensors, theidea is to sense one or more parameters of interest at one or morelocations, such as various levels of chemical concentration atgraduating depths in a concrete structure, and then read sensor data outalong with the device's identification code.

[0086] The ID code and digital response of device 104 also provides ameans of automatically logging data entry corresponding to the status ofeach sensor device. This may also include logging the correspondinglocation in the structure. In some cases, the position and depth of eachdevice 104 in a structure may be deduced from the amplitude and phasevariations of the communicated signal. These quantities vary with depth,material permittivity and permeability, moisture content, and proximityof re-bar, if any. The present invention may thus also includealgorithms that determine the depth and position of device 104 within astructure such as the depth and position of device in concrete asillustrated in FIGS. 1 and 2.

[0087] Various types of identification are known in the art and may beused with this invention. Examples of identification include microchipsstoring the ID code (e.g., an EPROM), magnetic sensor devices,electrical circuits providing a plurality of resonant circuits, and thelike. In some cases, the identification does not include a unique numberbut includes other information that may distinguish a device from othersimilar devices. By way of example, the device's known location may beused to distinguish it from other devices.

[0088] Wireless ID tags are commercially well known and there existsnumerous manufacturers that currently offer a wide selection of RFIDtags. These tags are either passive (typically operating near 125 kHz)or active (often operating near 2.45 GHz). Major manufacturers includeTexas Instruments of Dallas, Tex., Micron Communications of Boise, Id.,and Motorola of San Jose, Calif. Commercially available RF technology issuitable for use in many designs described herein.

[0089] Because applications in which the sensor device 104 areimplemented may vary considerably and may include environmentallyprohibitive conditions, specific features of device 104 may be governedby a particular application. For example, in a roadway health monitoringsystem, all components of the sensor device 104 a would be expected tosurvive temperatures that the road is exposed to. In various regions ofthe United States, bridge temperatures vary from −20 degrees Celsius to40 degrees Celsius.

[0090]FIG. 3B illustrates an electrical equivalent circuit 120 for thesystem schematized in FIG. 3A in accordance with a specific embodimentof the present invention. Inductive coupling between interrogator 132and sensor device 134 is represented in this circuit by mutualinductance 136.

[0091] Interrogator 132 includes leads 133 and 135 from an oscillator orsimilar circuitry that provide current at a particular frequency forantenna 138. In a specific embodiment, antenna 138 operates at 125 kHz.Leads 133 and 137 allow reader data detection circuitry to measure anyresponses received by antenna 138. Inductance 136 in the reader circuit132 is series resonated with capacitor 141 to maximize the currentthrough the reader antenna 138. This technique maximizes the magneticfield generated at the antenna 140 of sensor device 134.

[0092] In contrast, the inductance in sensor device circuit 134 isparallel resonated with capacitor 143 to maximize the voltage acrossantenna 140. This technique maximizes the peak RF voltage that isrectified to power producing microchip 142. The voltage produced bymicrochip 142 is provided to an output power supply switch 144, asensor, or other components of device 134.

[0093] In one embodiment, the design of antenna coils 138 and 140 isdriven by a desire to maximize mutual inductance between interrogator132 in sensor device 134. The mutual inductance between two coils may beapproximated by:

M ₂₁=[μ_(eff)μ₀ πR ² ₂ R ² ₁ N ₁ N ₂]/[2(D ² +R ² ₁) ^({fraction (3/2)})

[0094] although this mutual inductance depends on effectivepermeability, μ_(eff)μ₀, of the tag coil core and the distance betweencoils, D, noteworthy intrinsic coil parameters are its radius, R₁, andits number of turns, N₁. Thus, in one embodiment, coil 140 diameter andnumber of turns are as large as possible without exceeding spacelimitations of device 134 or causing coil 140 to be self resonant at theoperating frequency. In addition, self inductance of device 134 shouldnot be so large that resonating capacitor 143 is too small. In aspecific embodiment, resonating capacitor 134 is at least 10 pF.

[0095] Often the identifier is closely coupled to the transponder. Forexample, FIG. 3C illustrates typical components of a commerciallow-frequency “rice-grain” RFID sensor device 202 in accordance withanother embodiment of the present invention. Sensor device 202 mayinclude a ferrite-rod inductor 204 including a coil antenna, resonatingcapacitor 206, and silicon microchip 208. In one embodiment, the coilantenna is constructed using an air core and a ferrite rod. RF energy isinductively coupled to the RFID coil. When sufficient voltage isavailable, the microchip 208 is able to produce a sufficient rectifiedDC voltage to power the microchip. When powered, microchip 208 returnsits ID and by modulating the impedance of the resonating coil. Suitablemodulation schemes include ASK, PSK, and FSK for example. The componentsare conventionally connected together using bonding wire or rigid metalrails. The entire RFID, together with a sensor 214 and a memorycomponent 212, may be encapsulated in cement 210 or another suitableenvironmental protection material. This particular device is about thesize of a grain of rice, but other shapes are available commercially.

[0096] A sensor device of the present invention may respond with aquantitative indication of the parameter or with a specific state of thesensor. For example, a fuse may be used to measure corrosion of asurrogate thin wire that parallels corrosion of a metal element in astructure. If the wire corrodes and breaks, the wire failure event isdetected by a state change of the fuse. The failure event may then beused to signal significant corrosion in the metal. FIGS. 3D and 3Eillustrate two concepts for determining a sensor state from ID datareceived from a sensor device in accordance with two embodiments of thepresent invention.

[0097] The sensor device 180 of FIG. 3D communicates information relatedto a parameter being monitored based on a frequency shift. System 180comprises a microchip 182, resonating capacitors 183 and 184, fuse 185,and antenna 186. Resonating capacitors 183 and 184 are disposed inparallel and used to establish resonant frequency sufficiently far apartfrom each other. If fuse 185 is closed, the sensor device achieves itsmaximum response at one frequency. When fuse 185 is open, its maximumresponse is a different frequency. This allows sensor state system 180to provide binary feedback pertaining to whether a particular thresholdfor a parameter being measured has been met. In a specific embodiment,microchip 182 is a MCRF200 microchip as provided by MicrochipTechnology, Inc. of Chandler Ariz.

[0098] The sensor device 190 of FIG. 3E communicates information relatedto a parameter being monitored based on bit stream inversion. System 190comprises a microchip 192, capacitor 194, fuse 195, and antenna 196, allin parallel. System 190 provides binary feedback pertaining to whether aparticular threshold has been met according to an inversion of the IDcode of microchip 192 when fuse 195 is open. In this case, aninterrogator probing the sensor distinguishes the difference between thetwo codes produced from microchip 192. System 190 will also work if thefuse/switch is replaced by a suitable voltage level change, such as theoutput from a comparator. In a specific embodiment, microchip 192 is aMCRF202 microchip as provided by Microchip Technology, Inc. of ChandlerAriz.

[0099] In one embodiment, a sensor device of the present inventioncomprises an RFID technology and a sensor encapsulated in a pebble-sizedenclosure and embedded into the concrete structure. FIG. 3F illustratesa simplified cross-section view of device 50 in accordance with anotherembodiment of the present invention. Device 50 includes an encapsulation52 that defines an interior 53. Within interior 53 is sensor 54,electronics 56, ferrite coil 58, and coil windings 59. In a specificembodiment, device 50 of FIG. 3F is cylindrical and has a diameter inthe range of about ½ inch to about 2 inches and a height of about ½ inchto about 2 inches. In another specific embodiment, device 50 ischaracterized by a maximum dimension less than about 3 inches.

[0100] Encapsulation 52 seals internal components of device 50 from thesurrounding structure and environment. Encapsulation 52 also comprisesan inlet port 51 that allows intimate interface to the surroundingstructure. In one embodiment, port 51 is permeable to chloride ions andallows diffusion of chloride through port 51 for reception by sensor 54.Port 51 is comprised of a material corresponding to the parameter beingmonitored and specific application. For detection of chloride ionpresence in a particular concrete, port 51 may comprise any materialthat allows diffusion of chloride ions such as concrete or any otherpermeable cementitous material.

[0101] As will be described in more detail below, sensor 54 detects aparameter, such as a parameter associated with the health of a structurethat device 50 is embedded within. Sensor 54 may detect a simple binarythreshold level corresponding to a parameter of interest, or maygenerate an electric potential dependent on a parameter being monitored,e.g., the concentration of chloride ions received at sensing elements.

[0102] Electronics 56 may include one or more of the followingstructures: a) processing logic and electronics that convert a responsegenerated by sensor 54 into a suitable response signal for transmissionfrom device 50, b) receiving electronics that rectify an incoming probesignal from an interrogator to power one or more components of device50, c) measurement correction electronics that adapt measurementsprovided by sensor 54 and/or correct for any environmental conditions,if necessary, and d) identification electronics that allow device 50 tobe uniquely identified from an array of similar devices 50. Electronics56 may also include interface electronics between processing logic inthe device and the sensor. The interface electronics will depend on thesensing components. As shown with respect to the circuit diagram of FIG.4D for example, interface electronics between a chloride sensor 362 andmicrochip 364 include operational amplifier 368 and analog to digitalconverter 370.

[0103] In one embodiment, device 50 may be realized by integrating asensor with a passive commercial radio frequency identification (RFID)microchip. Such a microchip is also commonly referred to as a ‘tag’because of its widespread use in inventory control. Commerciallyavailable RFID technology may then be integrated with an array ofdevices 50 and interrogator designed to communicate with each device 50.In one embodiment, each device 50 comprises and RFID microchip such asthe MCRF200 or MCRF202 as provided by Microchip Technology, Inc. ofChandler Ariz.

[0104] In one embodiment, device 50 is designed for long-term inspectionapplications and one or more components of device 50 do not requirecontinual power for operation. In this case, energy may be supplied tothe sensor device using a wireless illumination, such as radio frequencyor microwave frequency illumination. Some sensor devices of the presentinvention require from about 5 microwatts to about 50 microwatts ofpower as supplied by an interrogator, and may consume energy on theorder of a few hundred nanojoules. Other sensor devices of the presentinvention require from about 10 microwatts to about 25 microwatts ofpower as supplied by an interrogator. Illumination of a chloride sensoras described with respect to FIG. 4C for example may result in availableenergy in the order of a few tens of microjoules.

[0105] In one embodiment, a sensor device of the present invention ispassive. As the term is used herein, passive refers to the notion thatthe device, or any of its components, do not rely on a local continualpower source, e.g., such as a battery, included in the device. Asdescribed above, power for responding to a wireless interrogation may betemporarily achieved using a wireless interrogation. Passive sensordevices that derive their power from RF illumination eliminate the needfor battery power and battery maintenance. In one embodiment, a passivesensor included in an embedded sensor device of the present does notmake a sensor reading until triggered by RF illumination.

[0106] The ability for devices of the present invention to operatewithout dedicated or internal power allows embedded sensor devices tosense and report data for extended periods up to, in some cases, thelifetime of a structure. For example, passive sensor devices of thepresent invention are well-suited for long-term inspection applicationssuch as inspection of chloride ingress into road structures. Bridgedesign life goals are about 75-100 years. Because sensor devices of thepresent invention do not use batteries or limited life power supplies,monitoring of bridges and extended longevity structures is indeedpossible.

[0107] Alternatively, concrete monitoring using devices of the presentinvention may include passive sensors that perform long-term and passivemonitoring of moisture, pH, chloride, and other environmental andstructural parameters of interest. Passive sensing also allows devicesas described herein to transpond data either in real-time or transponderthreshold events or exposures recorded in the past. It is contemplatedthat not all sensor devices of the present invention need be passive andsome may include an internal power supply such as a battery.

[0108] In one embodiment, sensor devices of this invention include someform of recording mechanism coupled to the sensor. The memory device mayrecord a physical or chemical event when the sensor provides anindication that the event has occurred. “Recording” usually means thatthe mechanism has changed. As discussed with respect to FIG. 3E forexample, the recording mechanism changes the device's resonancefrequency. For the device of FIG. 3E, a digital value in a memorylocation changes. The sensor device may be designed such that the statechange does not spontaneously reverse. Thus, when the physical orchemical event triggers a change from state 1 to state 2, the recordingmechanism remains in state 2 even after the physical or chemical eventceases or reverses back to state 1.

[0109] Alternatively, the sensor device may be designed such that thestate change reverses with a change of the parameter back to state 1.

[0110] The sensor device may also designed such that the state change isreversible. This is particularly useful for monitoring reversible andeffective extraction of chloride ions from roadways and bridge decks.Remediation strategies are available to extract chloride out of bridgedecks, such as electrochemical extraction. Thus, sensor devicesdescribed herein may allow a chloride sensor to be restored to aninitial state from an over limit state, thereby indicating that aremediation treatment has been effective.

[0111] When used, the recording mechanism should cause a sufficientchange in the operation of the sensor device to be detectable by thechosen interrogation means. In the case of a resonant circuit, forexample, the frequency shift recording the event needs to be measurable.Generally, the recording resonant circuit needs to change frequency byan amount greater than the width of the resonance (quality factor Q).Examples of recording mechanisms include electrical circuits,electromechanical circuits, mechanical latching mechanisms, programmableintegrated circuits such as EPROMs, fusible links, magnetic circuits,acoustic circuits, optical/IR circuits, and the like.

[0112] In some embodiments, a memory component includes multiplerecording mechanisms, all able to store and record different physical orchemical parameter levels or events. The different physical levels orevents may all pertain to the same parameter such as various chemicalconcentrations. For example, a chemical concentration detection systemmay include three separate recording components and/or sensors, eachconfigured to record a separate threshold chemical concentrationrelevant to monitoring chloride ingress into a bridge deck.

[0113] Some identifier tag/interrogation systems are designed to bepolled one at a time (serially), while other interrogators are able topoll multiple tags simultaneously. In practice, more than one sensordevice 50 may be an interrogator's interrogation field at a time. Toprevent collision of overlapping response signals, techniques may beimplemented to distinguish between different but adjacent, sensordevices. Communications techniques typically make use of anti-collisionand arbitration procedures that control the time when a tag responds toa probe. Different RFID manufacturers implement different anticollisionalgorithms. One approach is to have each sensor device transmit at arandom time slot and have the interrogator search different time slotsand reject multiple readings of the same tag.

4. Exemplary Sensor Designs

[0114] Specific sensors that may be used in some embodiments of thisinvention include sensors that detect or measure chemical or biochemicalspecies, temperature sensors, electrochemical cells that measure thepresence or level of an ion, pressure sensors, flow sensors,stress/strain sensors, accelerometers, dielectric sensors, conductivitysensors, shock sensors, vibration sensors, position sensors, sensorsthat detect thermal exposure, optical exposure, x-ray exposure,microwave exposure, pollutants, particle size, alignment, and the like.

[0115] Most any type of sensor may be used with this invention, so longas it meets the functional requirements. Sensors may be classified basedupon the parameters that they sense and the transduction mechanisms theyemploy. Very many sensor types are known and used for differentapplications. Many examples of things to be sensed and sensingmechanisms are described by Julian W. Gardner in “Microsensors:Principles and Applications,” John Wiley, 1994 (incorporated herein byreference in its entirety and for all purposes). Among the listed itemsare (1) thermal sensors: temperature, heat, heat flow, entropy, heatcapacity; radiation sensors: gamma rays, X-rays, UV, visible, IR,microwaves, radio waves; mechanical sensors: displacement, velocity,acceleration, force, torque, pressure, mass, flow, acoustic wavelength,amplitude; magnetic sensors: magnetic field, flux, magnetic moment,magnetization, magnetic permeability; chemical sensors: humidity, pHlevel and ions, concentration of gases, vapors and odors, toxic andflammable materials, pollutants; electrical sensors: charge, current,voltage, resistance, conductance, capacitance, inductance, dielectricpermittivity, polarization, frequency, and the like.

[0116] A transduction mechanism is usually used to convert the sensedparameter or stored event into an electrical signal. Suitabletransduction examples include electrochemical, conductometric (changesin resistance or conductivity), potentiometric, capacitive,amperometric, calorimetric, optical, resonant, fluorescent,piezo-electric, optoelectric, magnetooptic, surface-acoustic wave,magnetoresistive, superconductive, and other effects.

[0117] Conventionally, most sensors are stand-alone, directly powereddevices that provide continual measurements of the quantity beingsensed. These devices often require their own battery or wiring to acentral power source. For many applications of this invention, suchconventional active sensors are not suitable because power requirementlimits operational lifetime, or are embedded in an inaccessible locationof a structure that denies wired power. Many applications requireparameter detection at highly infrequent intervals in the range ofseveral readings per year. Thus, many sensors of the present inventionare passive and receive energy from the parameter being sensed or theenvironment that the sensor is implemented in.

[0118]FIG. 4A illustrates a simplified cross-section view of a chlorideion sensor device 140 in accordance with one embodiment of the presentinvention. Device 140 is embedded and surrounded by concrete 145. Asshown in FIG. 4A, device 140 comprises a chloride ion sensor 142,threshold detector 144, antenna 146, temperature sensor 148,encapsulation 149 and microchip 147. Encapsulation 149 seals thecomponents of device 140 from the exterior of device 140 and includesport 143. Port 143 allows chloride ions to pass between the exterior 145of device 140 and sensor 142.

[0119] Chloride ion sensor 142 detects the presence of chloride ions. Inone embodiment, chloride ion sensor 142 converts a chloride ion level,such as chloride ion concentration, to an electrical output such asvoltage. As shown, chloride ion sensor 142 detects a level of chlorideions that pass through port 143.

[0120] Threshold detector 144 compares one or more particular levels ofoutput from sensor 142 with a predetermined threshold for anapplication. If sensor 142 outputs a voltage, then threshold detector144 may be a comparator that compares the voltage from sensor 142 with apredetermined voltage. The predetermined threshold voltages correspondto one or more particular chloride ion concentration levels of interest.Typically, the particular levels detected by threshold detector 144 aredetermined upon implementation of device 140. For a comparator,threshold levels may easily be established and modified usingpredetermined voltages on the comparator. Threshold detector 144 mayalso include a memory component such as a latch that changes in some waywhen a threshold has been reached.

[0121] Microchip 147 responds to an interrogation signal as received byantennae 146 and includes instructions, circuitry and other facilitiesfor this purpose. Microchip 147 may comprise a logic device,microprocessor, and/or one or more conventional processors. Chip 147also prepares a signal to be transmitted from sensor device 140 inresponse to an interrogation signal. This may include modulating areading or response provided by chloride ion sensor 142 or thresholddetector 144, for example. Chip 147 may include a rectifying facilitiesthat rectify an incoming probe to power one or more components of device140 and identification that allows device 140 to be uniquely identifiedfrom an array of similar devices. In addition, microchip 147 may alsoinclude a DC power outlet that allows collected power to be distributedto various components in device 140.

[0122] Sensor device 140 also includes measurement correctionelectronics that adapt measurements provided by sensor 142 and/orcorrect for any environmental variability that may affect sensorperformance, if necessary. For example, temperature sensor 148 detectsthe temperature of device 140, which is usually the same as concretesurrounding device 140. Measurement correction electronics coupled totemperature sensor 148 convert output of temperature sensor 148 toappropriate modification of the signal provided by sensor 142, thresholddetector 144, or microchip 147. Temperature sensor 148 is passive andreceives heat energy from in concrete 145.

[0123]FIG. 4B shows an exemplary circuit diagram 150 corresponding tothe chloride ion sensor device 140 of FIG. 4A. As shown, sensor device140 includes micropower circuitry including operational amplifiers andcomparators are used to determine whether chloride concentrations havereached a predetermined threshold.

[0124] Starting in the upper right hand side of circuit diagram 150,antenna 146 receives RF energy from an interrogator that probes device140. Initially, antenna 146 is connected to a DC rectifier 151 viaswitch 161. DC rectifier 151 provides energy to the electronics incircuit diagram 150. Capacitor 153 stores electrical energy collected byantenna 146 and provided by DC rectifier 151, and has an outlet thatprovides the energy at VDC. Timing circuitry 155 coordinates componentswithin circuit diagram 150 based on reception of a signal at antennae146.

[0125] As shown, reference electrode 156 and test electrode 157 areincluded in sensor 142 and quantify the amount of an ion that they aresubjected to in. Electrodes 156 and 157 output a voltage relative to theamount of the ion. Operational amplifier 158 converts the voltagedifference produced by the electrodes 156 and 157 to a level suitablefor comparison by comparator 159. A signal conditioner 160 may alter theoutput based on the temperature of concrete 145 sensed by temperaturesensor 148 of FIG. 4A.

[0126] Threshold voltage 162 produces a voltage that corresponds to athreshold chloride ion concentration of interest. Comparator 159compares the output of operational amplifier 158 with threshold voltage162. A logical LO output from comparator 159 indicates that chlorideconcentrations received by sensor 142 are below the threshold and withinacceptable ranges. In this case, and RFID 154 is energized at theappropriate time interval to allow the sensor to stabilize. A logical HIfrom comparator 159 indicates that chloride concentration is above thepredetermined threshold. In this case, RFID 164 is selected by switch163, transponding a different ID code from device 140 to aninterrogator. In this manner, two separate identification numbers areproduced from device 140 based on the amount of chloride detected.Alternatively, comparator 159 output may be used to modify the responseof a single RFID, thereby simplifying circuit 150.

[0127] In general, a variety of sensors may be used in the wirelessdevices of this invention. The sensor chosen for a particularapplication should be able to detect the physical or chemical parameteror event under consideration. Thus, the sensor should detect a change inthe parameter or parameters associated with the one or more physicaland/or chemical events. Further, the sensor should have a dynamic rangethat covers the physical and chemical events in question. The sensorshould also be able to withstand the operating conditions to which itwill be exposed and fit within good design practices includingreliability, accuracy, size, weight, safety, and compatibility withother components and the application. In one embodiment, anelectrochemical cell is employed to detect levels of a chemical speciesin concrete.

[0128]FIG. 4C illustrates an electrochemical cell sensor device 300 inaccordance with one embodiment of the present invention. Electrochemicalcell sensor device 300 is embedded in a structure 301 comprisingconcrete 303 or any other cementitious material. Electrochemical cellsensor device 300 measures the concentration level of chloride ions inconcrete 303 and transmits a wireless signal representative of themeasurement.

[0129] Electrochemical cell 302 is an electrochemical sensor thatgenerates an electric potential dependent on the concentration of an ionreceived at electrodes 304 and 306, such as chloride ions. Theelectrochemical cell 302 consists of two electrodes: an ion selectiveelectrode 306 and a reference electrode 304. The output quantity ofelectrodes 304 and 306 is a potential difference between the twoelectrodes.

[0130] Ion selective electrode 306 allows the voltage difference betweenelectrodes 304 and 306 to change with the amount of an ion received atits active element. In one embodiment, ion selective electrode 306 is anAg/AgCl electrode that responds to the presence of chloride ions. Asshown, ion selective electrode 306 includes silver (Ag) wire 312, ionselective active sensing element 314, and a lead 315 in electricalcommunication with one or more electrical components of sensor device300. Silver wire 312 is coupled on one end to active sensing element 314and to lead 315 at another end. Silver wire 312 allows electricalcommunication between sensing element 314 and lead 315. Active sensingelement 314 comprises silver chloride and is disposed in electrolyte316, which provides a medium for movement of chloride ions. In aspecific embodiment, the AgCl is melted onto the active sensing element314. Electrolyte 316 maintains electrical connection between ionselective electrode 306 and reference electrode 304. Electrolyte 316 iscontained and sealed within an outer shell 321 and comprises a saturatedsolution of calcium hydroxide, or any other suitable charge carryingaqueous liquid or solution. Permeable cementitous material 318 leads toan external port of sensor device 300 and allows communication ofchloride ions between concrete 303 and electrolyte 316.

[0131] Ag/AgCl is a suitable chloride sensing electrode. Hg/HgCl mayalso be used, but hazards associated with mercury make it undesirablefor some applications. Sulfide (S^(═)) could be sensed using a Ag/Ag₂Selectrode, pH could be sensed using nickel/nickel oxide oriridium/iridium oxide, sulfate could be sensed using Hg/Hg₂SO₄, etc.Electrodes are generally chosen to be sensitive to the ions to be sensed(chloride ions in this case) and insensitive to ions that could confusethe measurement. These choices are well-known to those skilled in theart of electrochemical measurement. For example,http://www.topac.com/ISE.html lists a variety of commercially availableelectrodes designed to detect specific ions. Although these electrodesare not consistent with the size and geometry of many devices describedherein, a sensor devices could be constructed to sense any of thesequantities using the proper electrode material.

[0132] Healthy concrete has a high pH, in the order of about 12-13. Asconcrete matures over its lifetime, the pH decreases due toenvironmental effects such as carbonation. Sensor devices of the presentinvention are substantially insensitive to pH changes in concrete andother structures. In one embodiment, electrolyte 316 contains an excessof calcium hydroxide to maintain substantially constant pH forelectrolyte 316.

[0133] Reference electrode 304 provides context for the voltagedifference between electrodes 304 and 306. In one embodiment, referenceelectrode 304 is a copper electrode. As shown, reference electrode 304includes copper (Cu) wire 320, electrolyte 322, permeable cementitiousmaterial 324 and lead 317 in electrical communication with one or moreelectrical components 305 of sensor device 300. Copper wire 320 iscoupled on one end to lead 317. Electrolyte 322 is contained and sealedwithin a containment shell 323 and provides a medium for the storage andmovement of ions provided through permeable cementitious material 324.Electrolyte 322 is saturated with copper sulphate (CuSO₄) and calciumhydroxide, or any other suitable charge carrying aqueous liquid orsolution. In one embodiment, electrolyte 316 contains an excess ofcalcium hydroxide to maintain substantially constant pH for electrolyte322.

[0134] In operation, chloride ions (Cl⁻) penetrate inlet membrane 318from the surrounding concrete 303. The chloride ions collect inelectrolyte 316. The Ag/AgClion selective electrode 306 is reversible tochloride ions according to the Nernst equation, and hence its potentialdepends on the activity of chloride ions. The potential of thiselectrode is measured with the respect to the reference electrode. Thevoltage between 304 and 306 is proportional to the logarithm of thechloride concentration. The voltage between electrodes 304 and 306changes and may be measured via a resistor included in electronics 305.

[0135] While electrochemical cell sensor device 300 has been describedwith respect to measuring chloride ions, ion selective electrode 306 maybe configured or designed to detect the presence and concentration ofany halide and is not limited to chloride ion selectivity. In addition,ion selective electrode 306 may be designed such that it is selective toany ion—particularly one that facilitates corrosion of a metal—and isnot limited to halide ion selectivity.

[0136] Chloride ion sensors of the present invention, such aselectrochemical cell 300 of FIG. 4C, may be configured to detect a widerange of chloride ion concentrations. In one embodiment, sensor 302detects threshold chloride concentration in the range of about 10⁻³ toabout 10⁻⁵ (weight percentage of cement) in an environment with the pHranging from about 8 to about 14. In a specific embodiment, sensor 302detects threshold chloride concentration in the range of about 10⁻⁴(weight percentage of cement) in an environment with the pH ranging fromabout 9 to about 13. In another embodiment, personnel may set athreshold for chloride ion concentration greater than about 30milliMolars, as determined by an application.

[0137] In specific embodiments, a chloride ion sensor of the presentinvention is configured to detect chloride ions in the range of about 10milliMolars to about 100 milliMolars. Some sensors detect a thresholdthat corresponds to a corrosion initiation threshold in for a metal inconcrete. One such threshold is 30 milliMolar chloride ion forreinforcing steel in Flyash concrete. Another particular corrosioninitiation threshold for a metal in concrete corresponds to about 24milliMolars (about 0.857 kg/m³). Yet another particular corrosioninitiation threshold for a metal in concrete corresponds to about 33milliMolars for an admixture concrete. For some health monitoringapplications, the accepted threshold value for chloride ion ingress isabout 0.014% chloride ion by weight percent of cement. Obviously,sensors of the present invention may be designed to detect a wide rangeof chloride ion levels. One of skill in the art will appreciate that aparticular chloride ion level will vary on a wide variety of factorssuch as the metal material composition, concrete type, etc. Otherfactors that may affect design parameters for a chloride sensor of thepresent invention include the typical chloride concentrations expectedto be seen in the structure, the rate of chloride diffusion in thestructure, the sensor device characteristics such as power availability,and other system constraints such as temperature and pressure of thesurroundings.

[0138] Thus, electrochemical cell within sensor device 300 measures apotential difference between a reference electrode 304 and an ionselective electrode 306. Leads 315 and 317 from reference electrode 304and ion selective electrode 306 are in electrical communication withelectronics 305. Electronics 305 include memory 310 and a transponder308. Memory 310 acts as an identification source that uniquelyidentifies the device, and is in electrical communication withtransponder 308. Here, ‘unique’ is relative to other similar sensordevices 300 embedded in structure 301. In one embodiment, memory 310also stores a second unique number for sensor device 300 that indicateswhen electrochemical cell 302 has detected a threshold potential levelfor chloride ions in concrete 303. The memory may be a stand alonedigital memory source or included in a microchip that that rectifies asignal provided to transponder 308. One suitable microchip for thesepurposes is a MCRF202 microchip as provided by Microchip Technology,Inc. of Chandler Ariz. When triggered by a wireless interrogationsignal, the transponder transmits a wireless signal that indicates thepotential difference status between electrodes 306 and 304, and includesinformation from memory such as a unique number for sensor device 300.Since device 300 is embedded, the signal is normally sent through aportion of concrete 303.

[0139] Electrochemical cell sensor device 300 is passive. This impliesthat all components in sensor device 300 do not require dedicated powerstored in device 300. The electrochemical cell for example relies on aconcentration gradient to provide sensing energy. Operation of passivetransponders and energy capture from an incoming interrogating signalhas been described above.

[0140]FIG. 4D illustrates a representative potentiometric thresholdmeasurement circuit 360 for the electrochemical cell sensor device 300of FIG. 4C in accordance with one embodiment of the present invention.Circuit 360 comprises electrodes 363 and 365, microchip 364, antenna366, operational amplifier 368, serial analog to digital converter (ADC)370, and temperature compensation circuitry 372. All the components ofcircuit 360 except electrodes 363 and 365 are sealed within an interiorcavity of sensor device 300, as indicated by seal 374. Electrodes 363and 365 may each be sealed in their own respective containment spaces,such as containment spaces 323 and 321 as described above.

[0141] Antenna 366 receives RF energy from an interrogator that probescircuit 360. Microchip 364 rectifies the RF energy and energizes theelectronics in circuit 360. Capacitor 365 stores electrical energycollected by antenna 366. Microchip 364 has an outlet 369 that provideselectrical energy. In a specific embodiment, microchip 364 produceselectrical energy at 5 microamps at five volts.

[0142] Electrodes 363 and 365 quantify the amount of an ion that theyare subjected to. Operational amplifier 368 converts the potentialdifference produced by electrodes 363 and 365 to a level suitable forcomparison by serial ADC 370. Serial ADC 370 converts the analog signalprovided by operational amplifier 368 to a digital signal.

[0143] In addition, serial ADC 370 includes measurement correctionelectronics that alter output from electrodes 363 and 365 to correct forenvironmental variations. Exemplary environmental variations that may becompensated for by correction electronics included in a sensor deviceinclude temperature, pH and wetness. Devices of the present inventionembedded within concrete and roadways are expected to survivetemperatures from about −10 degrees Celsius to about 50 degrees Celsius.These temperatures should not compromise sensor performance. Temperaturecompensation circuitry 372 is then included to accommodate fortemperature differences in the surrounding structure. Temperaturecompensation circuitry 372 includes a temperature sensor that detectsthe temperature of concrete surrounding device 300, and outputs a signalthat corresponds to the sensed temperature. Temperature compensationcircuitry alters a reading provided by electrodes 363 and 365 accordingto temperature of device 300. Typically, the temperature of device 300corresponds to the temperature of the ambient material that device 300is disposed in.

[0144] ADC 370 includes comparative facilities that compare the outputof operational amplifier 368 with a threshold voltage, and factor in theoutput from temperature compensation circuitry 372. A switch 367 iscoupled to the output of ADC 370 and informs microchip 364 when thethreshold has been reached. In a specific embodiment, microchip 364 isan MCRF202 as provided by Microchip Technology, Inc. of Chandler Ariz.This chip is capable of indicating when a threshold level has beenexceeded by inverting the ID code bitstream, and is able to powerelectronics within circuit 360.

[0145]FIG. 4E illustrates a vertical cross section view of a verticallycylindrical sensor device 400 in accordance with one embodiment of thepresent invention. Sensor device 400 comprises housing 402, atransponder, two electrochemical electrodes 406 and 408 contained inseparate reservoirs 407 and 409, respectively, and board 410 including alogic device.

[0146] Housing 402 provides physical protection for internal componentsof sensor device 400. This includes sealing the components from moisturein the ambient structure. Housing 402 includes an interface 404 thatallows coupling or attachment of device 400 to rebar, tendons, or othermetal elements in a structure. In a specific embodiment, housing 402 ismade from molded plastic. An encapsulation comprising concrete may alsobe disposed around housing 402 to make the external appearance of device400 resemble a common concrete pebble.

[0147] Sensor device 400 includes structural members for separating andcontaining various elements of sensor device 400. Lining 405 contributesto an aqueous seal and fluid containment for reservoirs included indevice 400 and comprises cylindrical lining portions 405 a and 405 b.Cylindrically lining portion 405 a contributes to an aqueous seal andfluid containment for reservoir 409. Disposed within reservoir 409 iscylindrical lining portion 405 b, which contributes to an aqueous sealand fluid containment for reservoir 407. Cylindrical lining portions 405a and 405 b are both attached to a bottom portion 405 c of lining 405.Lining 405 may be made of rubber or teflon, for example.

[0148] The transponder in sensor device 400 comprises ferrites 412,spacer 413, and wire 414. Ferrites 412 are cylindrical and surroundcylindrical rubber lining portion 405 b. In one embodiment, ferrites 412comprise a low loss conductor with high magnetic permeability, such as aceramic/metal mixture. Ferrites 412 help focus electromagnetic energybetween the interrogator and sensor device. As shown, two cylindricalferrites are used in device 400. Spacer 413 is cylindrical and surroundsferrites 412. Spacer 413 reduces the capacitance of the coil for wire414 such that it resonates at a higher frequency. Wire 414 is woundaround spacer 413 and ferrites 412, and acts as an antennae that sensesa magnetic field. The transponder in sensor device 400 is strong enoughto communicate (listen and transmit) through concrete between device 400and an associated interrogator. For example, read ranges of at leasttwelve inches are possible with device 400. In one embodiment, wire 414has an outside diameter from about ½ inch to about 4 inches and betweenabout 100 and 500 turns. In a specific embodiment, wire 414 is comprisedof 42 gauge copper and has an outside diameter about 1¼ inches and about300 turns while ferrites 412 is a 1¼ inch outside diameter ferritetorrid as provided by Fair-Rite Products, Corp., Wallkill, N.Y.

[0149] Device 400 includes an electrochemical sensor that generates anelectric potential dependent on the concentration of chloride ionsreceived by the device. Electrodes 406 and 408 comprise an ion selectiveelectrode 406 and a reference electrode 408.

[0150] Reference electrode 408 provides context for the voltagedifference between electrodes 406 and 408. As shown, reference electrode408 comprises copper lead 421 disposed in an electrolyte 424 saturatedwith copper sulfate. Electrolyte 424 may comprise any suitable solutionsuitable for carrying charge and is not limited to a copper sulfatesolution. Electrolyte 424 is contained within in reservoir 407 andprovides a medium for the movement of ions provided through cementitiousmembrane or plug 425. The volume of reservoir 407 is defined by rubberlining 405 b and the lower surface of cementitious membrane 425.Cementitious membrane 425 is permeable to silver ions and allowscommunication of silver ions between electrolyte 427 and electrolyte424.

[0151] Ion selective electrode 406 allows the voltage difference betweenelectrodes 406 and 408 to change with the amount of chloride ionsreceived at its active element. Ion selective electrode 406 comprises asilver chloride (AgCl) lead 421 that responds to the presence ofchloride ions as the electrode is reversible to chloride ions. Lead 421is disposed in an electrolyte 427 comprising calcium hydroxide in water.Electrolyte 427 may comprise any suitable solution suitable for carryingcharge and is not limited to calcium hydroxide in water. Electrolyte 427is contained within in reservoir 409 and provides a medium for themovement of chloride ions provided through cementitious membrane 425 anda medium for the movement of silver ions emitted from lead 421. One ormore chemicals may be added to electrolyte 427 to enhance ionconductivity. For example, copper sulphate added as a paste in reservoir409 may be suitable to enhance ion conductivity. The volume of reservoir407 is defined by rubber lining 405 b and the lower surface ofcementitious membrane or plug 429. Cementitious membrane 429 ispermeable to chloride ions and allows communication of chloride ionsbetween concrete the external environment and electrolyte 427.

[0152] As shown, membrane 429 is blocked by environment interfacemembrane 431, which controls inlet and outlet of chemicals and moleculesfor device 400. Membrane 431 permits selective ion inlet and preventsmoisture loss from device 400. Device 400 is often implemented in abridge where chloride ions travel faster when the bridge is wet. Sincethe bridge will subsequently dry, obtaining chloride ions and sensormeasurement is preferably performed during wet conditions. The devicethen provides data based on chloride penetration when the bridge is wetand ion travel is greatest, not at the time of interrogation when thebridge is dry. In a specific embodiment, membrane 431 comprises acementitious material (e.g., tile grout)

[0153] Electrodes 406 and 408 pass through rubber lining 405 and are inelectrical communication with board 410, e.g., via a solder connection.Rubber lining portion 405 c seals the passage of each electrodes 406 and408 therethrough. Board 410 allows electrical communication betweencomponents of device 400. In one embodiment, a resistor is disposedbetween electrodes 406 and 408 and the resistor produces a measurablevoltage as current flows between the electrodes. In this case, theresistor is disposed on board 410 as well. A logic device or microchipis also disposed on board 410 and is in electrical communication withelectrodes 406 and 408 and resistor. Functions of the logic device aredescribed above and not described herein for sake of brevity.

[0154] In operation, chloride ions (Cl⁻) penetrate inlet membrane 431and cementitious membrane 429 from the surrounding environment. Thechloride ions collect in electrolyte 427. With increasing concentrationof chloride ions in electrolyte 427, silver ions precipitate from AgCllead 427 into the calcium hydroxide solution. The silver ions build inconcentration and migrate through permeable membrane 425 and intoreservoir 407. With less silver ions on lead 427, the voltage betweenelectrodes 406 and 408 changes and may be measured via the resistordisposed on board 410.

[0155] In one embodiment, electrodes 406 and 408 are pre-treated (aged)so that electrochemical drift or temporal inconsistency affecting sensoroutput is substantially prevented during the operating lifetime of thesensor. This may be performed for example, by exposing the electrode 406to saturated calcium hydroxide solution (concrete pore solution istypically saturated Ca(OH)₂, so this effectively reproduces the initialbridge deck conditions) until any drift in electrode 406 performance hasbeen alleviated. A period of about one hour to about several hundredhours of ion exposure may be suitable in some cases. In a specificembodiment, electrodes 406 and 408 are exposed to chloride ions for aperiod of about one day to about fourteen days.

[0156] For a roadway or bridge deck application, the volume for device400 is derived from the size limitation on aggregates for high-strengthconcrete. Aggregate size is determined by many factors, including a)code requirements, b) thickness of the slab—aggregate size shouldgenerally be no more than about 25 percent of the thickness of the slab,and c) spacing of metal components. In some cases, aggregate size isdetermined by the desired compressive strength of the concrete. Higherstrength concrete require smaller diameter aggregates. In a specificembodiment, device 400 has a generally accepted standard of ¾ inches (20mm) diameter. A 20 mm diameter spherical aggregate has volume of 4.2cm³. For sensors of the present invention embedded in concrete when theconcrete is poured, the device may have a specific gravity that keeps itfrom floating in the concrete mix. In some cases, an encapsulationaround a sensor device of the present invention is compatible with thesurrounding concrete mix or cement matrix. Some factors in this regardinclude shape, roughness, impact hardness, and compressive strength.

5. Alternative Sensors

[0157] Although the present invention has primarily been described sofar with respect to chloride ion concentration detection, sensor devicesof the present invention may detect a wide variety of other parametersrelevant to a structure's health. FIG. 5 illustrates an organization ofsensor parameters useful for monitoring health of a structure inaccordance with various embodiments of the present invention. As shown,sensor devices 602 of the present invention may be characterized bythreshold or event detection 604 and parameter detection 606, such asreal-time detection of a parameter.

[0158] There are a wide variety of parameters, threshold, and eventsthat may be measured for sensors 602. As shown, threshold sensors 604and parameter sensors 606 may include concentration sensors 608, pHsensors 610, conductivity sensors 612, epoxy moisture ingress sensors614, corrosion of surrogate sensors 616, and polarization resistancesensors 618. One example of a concentration sensor 608 is a chloride ionsensor, which has been described in detail above. For example, a singlechloride sensor may include three threshold levels: a high concentrationlevel 622, a medium concentration level 624, and a low concentrationlevel 626.

[0159] Each of the sensors described in FIG. 5 may use a configurationsimilar to one of the devices described above. For example, FIGS. 6A-6Dillustrate various sensor devices that each detect a different parameteraccording to the sensor device embodiment shown in FIG. 4D. For eachcase, the device aside from the sensor remains substantially similar inbasic components while the sensor has been replaced, and in a few cases,a few processing elements have been added or changed.

[0160]FIG. 6A illustrates a representative circuit 640 for a sensordevice that detects conductivity in accordance with one embodiment ofthe present invention. Circuit 640 comprises a conductivity sensor,microchip 364, antenna 366, micro-power operational amplifier 644,comparator 646, and temperature compensation circuitry 372. Theconductivity sensor comprises four probes 650, 651, 652, and 654. Allthe components of circuit 640 except probes 650, 651, 652, and 654 aresealed within an interior cavity, as indicated by line 374. Chip 364,antenna 366, capacitor 365, seal 374, switch 367, and temperaturecompensation circuitry 372 are similar to that as described with respectto FIG. 4D.

[0161] Probes 650 and 651 are in electrical communication with amaterial whose conductivity is being measured, and provide a voltagedifference to micro-power operational amplifier 644 based on thematerial's conductivity. Micro-power operational amplifier 644 convertsthe potential difference produced by probes 650 and 651 to a levelsuitable for comparison by comparator 646. Probe 652 is ground as areference. Probe 653 is in electrical communication with currentlimiting resistor 648, which acts as a voltage reference for comparator646. Comparator 646 compares the output of operational amplifier 644with a threshold voltage, and factors in the output from temperaturecompensation circuitry 372. In one embodiment, comparator 646 is ananowatt comparator that receives a reference voltage of 1.8 volts fromcurrent limiting resistor 648. Probes 650, 651, 652, and 654 maycomprise stainless steel, copper, any metal that does not corrode in aconcrete environment, or any other suitable conductive element. A sensordevice comprising a conductivity sensor as shown in FIG. 6A isparticularly useful to measure the conductivity of metal elementsdisposed in a structure whose conductivity changes with corrosion.

[0162]FIG. 6B illustrates a representative circuit 660 for a sensordevice that detects pH in accordance with another embodiment of thepresent invention. Circuit 660 comprises a pH sensor, microchip 364,antenna 366, micro-power operational amplifier 644, comparator 646, andtemperature compensation circuitry 372. The pH sensor comprises twoelectrodes 662 and 664. Electrodes 662 and 664 are in ioniccommunication with a material whose pH is being measured, and provide avoltage difference to micro-power operational amplifier 644 based on thematerial's pH. Electrode 662 is a reference electrode, comprisingCu/CuSO₄ for example. Electrode 662 is an ion selective electrode,comprising Ni/NiO as an active element responsive to pH changes forexample. A sensor device comprising a conductivity sensor as shown inFIG. 6B is particularly useful to measure pH of concrete in a structurewhose pH changes over time.

[0163]FIG. 6C illustrates a representative circuit 680 for a sensordevice that detects epoxy moisture ingress in accordance with oneembodiment of the present invention. Circuit 680 is a conductivitysensor device similar to that as described with respect to FIG. 6Aexcept that it measure the conductivity of an epoxy sample 682. Somemetal elements in a bridge are protected by an epoxy disposed around themetal. In this case, a sensor device corresponding to representativecircuit 680 is included to detect deterioration of the epoxy.

[0164]FIG. 6D illustrates a representative circuit 690 for a sensordevice that detects multiple corrosion events in accordance with anotherembodiment of the present invention. System 690 comprises chips 692 and693, capacitor 694, wires 694 and 695, and antenna 696. All thecomponents save wires 694 and 695 are sealed from the externalenvironment as indicated by seal 691.

[0165] Wires 694 and 695 each act as a surrogate to a metal whosecorrosion is being monitored. Each wire 694 and 695 has a diameter thatindicates a corrosive level of interest. For example, wire 694 has adiameter D₁ and wire 695 has a diameter D₂, which is greater than D₁.Wires 694 and 695 are preferably the same material as the metal whosecorrosion is being monitored and corrode corresponding to the metalbeing monitored. However, at some level of corrosive exposure, each wire694 and 695 would corrode to the point at which it lost conductivity, orbecame open with respect to the microchip that it communicates with. Thelevel of interest may correspond to a corrosive level below fullcorrosion of the metal being monitored, or substantially equal to fullcorrosion, for example.

[0166] The device corresponding to circuit 690 transmits an ID inresponse to a probing signal to determine when either corrosion eventdetected by wires 694 and 695 has occurred. More specifically, circuit690 communicates corrosion of either surrogate wire 694 and 695 based onbit stream inversion of a microchip that each wire communicates with.Wire 694 is in electrical communication with microchip 692 and wire 695is in electrical communication with microchip 693. Chips 692 and 693provide binary feedback pertaining to whether wire 694 or 695 hascorroded, based on an inversion of the ID code of each chip 692 and 693when its respective wire 694 and 695 is open. In this case, aninterrogator probing the sensor device distinguishes between the codesproduced from each microchip. Suitable anticollision algorithms may alsobe used to prevent simultaneous response from each microchip 692 and693. In a specific embodiment, chips 692 and 693 are a MCRF202 microchipas provided by Microchip Technology, Inc. of Chandler Ariz.

6. Interrogators

[0167] An interrogator is used to probe a sensor device of thisinvention. The interrogator provides a wireless probe signal thattriggers the sensor device to respond with its identity and a sensorreading (e.g., a parameter status). In one embodiment, the signalprovided by the interrogator also provides the energy necessary for thesensor device to reply. The interrogator may be able to detect the replyand present that reply to a computer system or an individual conductingthe analysis. Note that devices performing the functions of (1)energizing the sensor device and (2) communicating with the sensordevice can be physically separate. They may use different signals forexample. In one embodiment, the device produces multiple responses andthe interrogator receives multiple signals. The first response may beignored by the interrogator and the second is used as the response. Thefirst response may be ignored due to uncertainty in the signal sinceelements in the sensor device may need time to reply (e.g., acapacitor).

[0168] As mentioned above, a wireless interrogation probe may take manydifferent forms such as an RF signal, a microwave signal, an electric ormagnetic field, etc. The transponder of the sensor device is designed torespond to type of signal provided by the interrogator. While it willoften be convenient to design the interrogator and the sensor'stransponder to send signals of the same type (e.g., both send RFsignals), this is not a requirement of the invention. For example, theinterrogator may provide a low-frequency magnetic field as a probe andthe transponder may deliver the sensor information via a microwavesignal.

[0169] An interrogator provides a probing signal (and power) to a sensordevice. Preferably the interrogator includes sufficient radiated powerto penetrate any structural material between the interrogator and thesensor device, sufficient radiated power to energize the device at thedesired read rates, sufficient bandwidth to interrogate the device in areasonable amount of time, sufficient sensitivity to accurately obtainthe device response, sufficient specificity to discriminate betweennearby devices (if desired based on the application), a suitableinterface to a computer to record and update a database of sensor devicehistory, a suitable size/weight/power limitation, suitable read range,and safety. An interrogator can accomplish sufficient radiated power toenergize the device by transmitting an electromagnetic (DC, wave orfield) or acoustic signal in the form of continuous wave, pulsed cirwave, chirped waveform, spread-spectrum waveform, impulse, or codedwaveform to energize the tag. A specific embodiment employs a commercialproduct such as that supplied by Biomark (Destron) with modification tomonitor sensed events. In one embodiment, conventional technology isused to produce the interrogator.

[0170]FIG. 7 illustrates an exemplary reader block diagram correspondingto an interrogator 500 in accordance with one embodiment of the presentinvention. Interrogator 500 includes a receiver antenna 502 that iscapable of receiving the resonant frequency of the one or more sensordevices it is polling. The interrogator 500 also includes a transmitantenna 503 that is capable of sending a suitable signal to the one ormore devices it is polling. The transmit antenna 503 and the receiverantenna 502 may be combined if suitable isolation circuitry is used. Thetransmit frequency used to query the sensor device may also be used as alocal oscillator in the homodyne receiver as illustrated. As mentionedin the description of FIG. 3A, a passive sensor device may rectify anincident RF signal coming from interrogator 500 to provide DC power forthe microchip 342.

[0171] Once the microchip is activated, it modulates the incidentcarrier with the proper ID code and provides a modulated backscattersignal. The response signal may be at a frequency different from that ofthe incident signal. A remote receiver detector 504, which may becoupled to the interrogator, detects this modulated backscattered signaland records the ID information using recorder 506. Interrogator 500 maybe used in conjunction with RFID sensor device 50 for example, in whichthe sensor device is capable of providing a differential frequencyresponse for varying memory states of the sensor device 50. In thiscase, interrogator 500 is capable of receiving at a plurality ofresonant frequencies (e.g., 103 kHz and 156 kHz). In addition,interrogator 500 includes one or more lowpass filters 508 as well as thedetector 504 which are coupled to controller 510 and computer interface512.

[0172] The choice of an operating frequency or frequencies may varywidely. For large arrays of sensor devices, regular or slightly modifiedcommercially available equipment may provide cost savings. These devicesoperate in designated frequency bands such as 125 kHz, 13.56 MHz, 900MHz, 2.45 GHz and 5.8 GHz. In some cases, RFID technology at 125 kHz isused due to its current maturity. Alternatively, it may be desirable toincrease the interrogation frequency to increase the data rate andinterrogation speed. Other criteria that may be used to select afrequency include penetration through structure materials such asconcrete and other lossy, conductive or non-conductive media, improvedread ranges and weight reduction.

[0173] In another embodiment of the present invention, interrogator 500is configured to interrogate multiple sensor devices simultaneously. Inthis manner, interrogation of a large number of sensor devices may beexpedited. For example, anti-collision RFIDs or algorithms that improvethe ability of the interrogator to read multiple sensor devices are alsosuitable for use with the present invention. By way of example, a timedomain multiple access (TDMA) system may be used in which a passivesensor responds with a time delay to interrogation.

[0174] A hand-held interrogator may also be used (FIG. 2A), and may pollmultiple devices simultaneously. In another embodiment, the interrogatoris non-stationary and transported by a vehicle such as that describedwith respect to FIG. 2B. For roadway inspection, having a vehicle carrythe interrogator allows interrogation to occur without highway personnelleaving the vehicle and for interrogation to occur at moving vehiclespeeds—further simplifying inspection and reducing inspection time.

[0175] It is also possible to use multiple interrogators to speed orotherwise improve inspection. These may be carried by multiple vehicles,or multiple interrogators may be placed on a single vehicle, forexample. Regardless of the interrogator used for an application, theinterrogator should have a suitable read range for probing the array ofdevices. By way of example, a hand held interrogator may have a readrange from 1 inch to 12 inches. Higher read ranges also permit moredevices to be probed simultaneously. Generally speaking, increased readranges may be obtained by the use of increased interrogator power,increased size of the interrogator transmit antenna, increased size ofthe sensor device antenna, low power sensor device design (such as usingcomponents that make use of 3V logic instead of 5 V logic), increasedsize of the receiving antenna, and the use of shielding and interferencemitigation strategies to improve reception capabilities and minimizesignal leakage in the interrogator. In some cases, certain modulationand coding schemes for transponding data perform better in a noisyenvironment and these techniques are generally well known to thoseskilled in the art.

[0176] Regardless of the interrogator used, the interrogation processmay be tuned to application specific requirements or to overcomeapplication specific obstacles. Such obstacles include narrow-band andbroad-band interference. To overcome weak signal reception from sensordevices in the presence of strong reader transmission, the interrogatormay transmit short pulses and listen for sensor device echo when thetransmitter is off. Alternatively, the interrogator may implement asequence of isolation strategies to separate the receiver from thecontinuous wave transmission emissions. Physical separation, placementin pattern nulls and orthogonal polarization can achieve separation, forexample. Transceiver-receiver isolation may also be achieved by the useof a high dynamic-range amplifier and mixer components and signalsubtraction.

[0177] The general procedure involved in sensing and interrogation inaccordance with this invention is depicted as process flow 800 of FIG.8. The process flow 800 of FIG. 8 is particularly useful for monitoringthe monitoring the health of a bridge comprising concrete and a metal.

[0178] Process flow 800 begins by embedding one or more sensor devicesin the structure (802). The sensor device comprises a sensor thatdetects a parameter indicative of the health of the structure, anidentification source that can distinguish the device from the othersimilar devices, and a transponder. In one embodiment, sensor devices ofthe present invention are employed within established roads and bridgesby embedding the devices in back-filled cores (see FIG. 2B). In thiscase, the embedded sensors are inserted in the side wall of the corehole at various depths and permit estimation of chloride ingress atthese depths. The core may be refilled with low porosity concrete (e.g.,comprising polyester concrete) such that the sensors may measurechloride ingress through the extant structure. The sensor devices mayalso be embedded within new concrete to provide internal monitoring ofchloride ion ingress and other chemical species penetration and attack.Immediately after installation in a new structure, the sensor devicesshould all respond negatively, that is, chloride concentration is belowthreshold limitations established for each device.

[0179] Subsequently, a sensor included in the device detects a parameterstatus (804) and is probed by an interrogator (806). This may occur inany order. For example, the sensor may make a sensor reading in responseto a wireless probe from the interrogator. Alternatively, the sensor mayperiodically adapt based on the parameter being sensed (e.g., anevolving chemical change), and the interrogation signal probes thedevice at a later time. The interrogation signal may penetrate portionsof the structure, as determined by the sensor device's position.

[0180] In response to the probe, the device returns a wireless signal(808). Similarly, the return signal may transmit through a portion ofthe structure. The return wireless signal indicates the parameterstatus. The return signal may also include an identification for thedevice. In one embodiment, the power incident on the sensor device isrectified to produce DC power used to operate the sensor device. Next,backscatter from the sensor device antenna (coil) is modulator by anRFID chip according to the ID code and sensor state in the sensor devicememory. The transceiver demodulates the received backscatter and reportsthe ID and sensor reading to an associated computer. If desired, thecomputer may then update a database for the sensor device beingmonitored and flag a particular structural location corresponding to thesensor device position for further inspection and/or maintenance.

[0181] In one embodiment, a hand-held interrogator illuminates a localregion, powering any embedded sensors in the region and obtaining datafrom the sensors. In another embodiment useful for bridge inspections,the interrogator is carried by a truck, or any other moving vehicle, anddriven over the sensor.

[0182] If the system is provided with an array of sensor devices, anevent or threshold may be proximate at least one of the sensor devices,which detects the physical or chemical event, while other sensor devicesin the array do not detect the event. In some cases, after a sensordevice is exposed to a condition of interest, it passively records theevent using a recording mechanism. For example, the wires 692 and 693may passively record corrosion levels by their irreversible corrosionand open circuit deterioration.

[0183] The interrogator may note the sensor information and otherrelated information. If the reporting device is one of a group ofrelated devices, the interrogator system may retrieve informationidentifying the spatial or temporal position of the reporting devicewithin the group. The retrieved information may be provided in adatabase in which device location is keyed to device identification (IDtag information). Such a database is depicted within a system in FIG. 9.

[0184] Sensing (804), probing (806) and data return (808) may berepeated as desired by an application. Bridges for example are inspectedbiennially. Regular inspection also allows chloride ingress in a bridgeto be tracked. Returning to FIG. 2B, is possible at a subsequent timethat the top device 50 a indicates excess chloride concentration, butthe remaining devices 50 b-50 d indicate acceptable chlorideconcentration levels. The date of this measurement along with the knownlocation of each device 50 provides a first estimate of the diffusionconstant for this roadway or bridge deck. By correlating thisinformation with known sensor depth and past history, chloride ingressprogress may be tracked. As inspection continues over the years, highwaypersonnel may track continued progress of chloride ingress as devices 50b-50 d deeper into roadway indicate chloride over limit conditions. Andif chloride concentrations reach an over limit condition for sensordevice 50 d, more conclusive or additional action and may then bescheduled by highway personnel.

[0185] Applying this information collection over an entire bridge andhundreds of other bridges allows maintenance technicians to prioritizemaintenance schedules for a large number of bridges. In addition, basedon diffusion rate predictions from historical measurements, bridgetreatment of hundreds or thousands of bridges in an area may beprojected and prioritized based on quantitative data. Further, embeddedsensors that provide chloride ion concentration measurement over timemay also be used for detailed monitoring, regional comparisons, andresearch.

[0186] Sensor devices of the present invention may also monitorreversible and effective extraction of chloride from roadways and bridgedecks. Remediation strategies are available to extract chloride out ofbridge decks, such as electrochemical extraction of chloride.

[0187] The interrogation process illustrated in FIG. 8 is appropriatefor some applications calling for a course inspection of a largestructure followed by a fine examination of selected regions. Theinterrogation process involves a collection of devices (e.g., an arrayon a large structure with many devices such as a bridge. Initially, thestructure is probed quickly to make sure that all devices are presentand actually functioning. Next, the interrogator determines if any ofthe devices was exposed to the event of interest. This procedure may beperformed without identifying specific devices in the collection. Theinterrogating signal may be chosen to identify frequencies that arecharacteristic of significant events, probing the entire structure (orat least a large region) all at once. The interrogator determineswhether a “bad” response was detected. If such a bad response isdetected, a more detailed inspection may be needed to determine thelocation of the device. If not, the process is complete. If theprocedure finds that at least one device was exposed to the condition ofinterest, then a more involved interrogation may be performed, such as amanual inspection according to conventional analysis of core a sample.

[0188] The interrogation process illustrated in FIG. 8 may also includecross-referencing with a database, memory or other information storagemechanism. The database may be useful for storing information for adevice based on its ID including the device's physical location orsensing history. A database is particularly appropriate when an array ofsensor devices is probed.

[0189] In one embodiment to determine the location of the respondingdevice, interrogator (or a related system) queries a database containinga list of device IDs and corresponding spatial locations. Database thenresponds with the location of the device identified in the query. Thisembodiment is particularly useful when the system includes an array ofdevices and interrogator determines which specific device within thearray is reporting its status.

[0190] A cost-effective highway maintenance program may establishpriorities based on the condition of each roadway and bridge beingmonitored. However, condition based maintenance requires routine roadwayinspection to monitor degradation of the roadway and bridges surfacesand subsurfaces. When numerous devices are implemented in each roadwayand bridge, the identification and state of each device may be stored ina database or otherwise recorded. The database allows a history for eachdevice to be maintained. In the case of roadway inspection, the databasealso provides simple tracking of bridge health for a large number ofbridges.

[0191] In a roadway and bridge health monitoring application, eachindividual sensor device may be identified as belonging to a uniquelocation or known location relative in a roadway or bridge. In thisapplication, where hundreds or thousands of individual devices may beimplemented, the ID tag may include a 16 or 18 bit signal in which somebits are reserved for the sensor information and data and the remainingbits are reserved for identification of the individual device.

[0192] A computer-implemented user interface may be used to improve useranalysis of an array of sensor devices. By way of example, a graphicaluser interface (GUI) may be used to help inspectors with roadwayanalysis. The roadway GUI may contain different colors for uninspecteddevices, inspected devices that read a particular concentrationthreshold, and inspected devices that did not record an event. Inaddition, the GUI allows the user to point a particular device in anarray and obtain information on the device. The information may includethe device's ID, location in the structure, and any history for thedevice as stored in a database. Any suitable GUI system suitable forintegration with the reader coil may be used. In one specificembodiment, the GUI is implemented on a portable computing device (suchas a Palm Pilot (3Com Corporation of Santa Clara, Calif.) or the like)to allow a user to view both the item under inspection and the computerdisplay of the item.

10. Conclusion

[0193] The versatility, small size and wireless unobtrusive nature ofthe inventive remote sensor devices allow for sensor application in manyinteresting applications. Coupling the transponderand the sensortogether allows for reduced weight and size. Further, the identificationmeans of the inventive sensor devices allow for monitoring of systemswhere potentially thousands of sensors are implemented and each sensormay be individually monitored. While this invention has been describedin terms of several preferred embodiments, there are alterations,permutations, and equivalents that fall within the scope of thisinvention which have been omitted for brevity's sake. It is thereforeintended that the scope of the invention should be determined withreference to the appended claims.

What is claimed is:
 1. A sensor device comprising: a sensor that detectsa parameter indicative of the health of a structure comprising a metal;a transponder in electrical communication with the sensor and thattransmits a wireless signal through a portion of the structureindicating the parameter status when triggered by a wirelessinterrogation signal; and an identification source in electricalcommunication with the transponder that uniquely identifies the device.2. The device of claim 1 wherein the device does not contain a powersource.
 3. The device of claim I wherein the sensor detects one of: theresistance of a metal element included in the structure, concentrationof an ion found in the structure, pH in the structure, conductivity of ametal element included in the structure, and corrosion of a surrogate.4. The device of claim 1 wherein the device is embedded in thestructure.
 5. The device of claim 1 wherein the sensor detects the levelof a chemical that is aggressive to the health of the structure.
 6. Thedevice of claim 5 wherein the sensor detects the level of an ion thatfacilitates corrosion of the metal.
 7. The device of claim 1 furthercomprising temperature compensation circuitry that alters a sensorreading according to temperature of the device.
 8. The device of claim 1wherein the identification source comprises a microchip that stores aunique number for the sensor device.
 9. The device of claim 8 whereinthe microchip also stores a second unique number for the device thatindicates when the sensor has detected a threshold level for theparameter being sensed.
 10. The device of claim 9 wherein the thresholdcorresponds to a chloride ion concentration.
 11. The device of claim 1wherein the transponder comprises: a ferrite; and at least one wirewound around the ferrite.
 12. The device of claim 1 wherein the deviceincludes a microchip that rectifies a signal provided to thetransponder.
 13. The device of claim 1 wherein the sensor comprises anelectrochemical cell.
 14. The device of claim 1 wherein the sensordetects one of humidity, temperature, and pressure.
 15. The device ofclaim 1 further comprising a memory component that allows for recordingan event detected by the sensor.
 16. The device of claim 15 wherein theevent is a threshold for the parameter.
 17. The device of claim 1further comprising an amplifier for increasing the strength of areceived incident signal, a modulator for modifying the incident signalwith information provided by the sensor, and an antenna or antennas forreceiving and transmitting the incident signal.
 18. The device of claim1 wherein the device communicates information related to the parameterbeing detected using a bit stream inversion.
 19. A device embedded in astructure comprising concrete, the device comprising: a sensor embeddedin the concrete that detects a parameter; a transponder in electricalcommunication with the sensor and that transmits a wireless signalthrough a portion of the concrete indicating the parameter status; andan identification source in electrical communication with thetransponder that uniquely identifies the device.
 20. The device of claim19 wherein the transponder transmits the wireless signal when triggeredby a wireless interrogation signal provided by an interrogator.
 21. Thedevice of claim 19 wherein the structure comprises a metal.
 22. Thedevice of claim 21 wherein the sensor detects a parameter indicative ofthe health of the structure.
 23. The device of claim 22 wherein thesensor detects a parameter indicative of the health of the metal. 24.The device of claim 23 wherein the sensor detects the presence of an ionthat facilitates corrosion of the metal.
 25. The device of claim 24wherein the structure comprises concrete and the sensor detects chlorideion concentration in the concrete.
 26. The device of claim 19 whereinthe structure is included in a bridge.
 27. The device of claim 19wherein the sensor and transponder are passive.
 28. The device of claim19 wherein the sensor detects one of: resistance of a metal elementincluded in the structure, concentration of an ion found in thestructure, pH in the structure, conductivity of a metal element includedin the structure, and corrosion of a surrogate.
 29. The device of claim19 further comprising temperature compensation circuitry that alters asensor reading according to temperature of the device.
 30. The device ofclaim 19 wherein the identification source comprises a microchip thatstores a unique number for the device.
 31. The device of claim 30wherein the microchip also stores a second unique number for the devicethat indicates when the sensor has detected a threshold level for theparameter being sensed.
 32. The device of claim 31 wherein the thresholdcorresponds to a level of chloride ion concentration in the concrete.33. The device of claim 1 wherein the transponder comprises: a ferrite;and a wire wound around the ferrite.
 34. The device of claim 19 whereinthe device includes a microchip that rectifies a signal provided to thetransponder.
 35. The device of claim 19 wherein the sensor comprises anelectrochemical cell.
 36. A device comprising: an electrochemical cellthat measures the potential difference between a reference electrode andan ion selective electrode; an identification source that uniquelyidentifies the device; and a transponder in electrical communicationwith the electrochemical cell and in electrical communication with theidentification source that transmits a wireless signal indicating thepotential difference and information from the identification source. 37.The device of claim 36 wherein the device includes a microchip thatrectifies a wireless signal provided to the transponder.
 38. The deviceof claim 36 further including a permeable cementitious membrane thatpermits passage of an ion from outside the device to the ion selectiveelectrode.
 39. The device of claim 36 wherein the ion is a halide. 40.The device of claim 39 wherein the halide is chloride.
 41. The device ofclaim 36 wherein the ion selective electrode is selective to an ion thatfacilitates corrosion of a metal.
 42. The device of claim 36 wherein theion selective electrode is an AgCl electrode.
 43. The device of claim 36wherein the electrochemical cell measures the voltage difference betweenthe ion selective electrode and a copper electrode.
 44. The device ofclaim 36 wherein the transponder transmits a wireless signal through aportion of a structure that the device is embedded in.
 45. The device ofclaim 44 wherein the structure comprises concrete and theelectrochemical cell detects chloride ion concentration in the concrete.46. The device of claim 36 wherein the sensor and transponder arepassive.
 47. The device of claim 36 further comprising temperaturecompensation circuitry that alters a sensor reading according totemperature of the device.
 48. The device of claim 1 wherein theidentification source comprises a microchip that stores a unique numberfor the device relative to other similar devices.
 49. The device ofclaim 48 wherein the microchip also stores a second unique number forthe device that indicates when the electrochemical cell has detected athreshold potential level.
 50. The device of claim 49 wherein thethreshold level corresponds to a level of chloride ion concentration.51. The device of claim 36 wherein the transponder comprises: a ferrite;and a wire wound around the ferrite.
 53. The device of claim 36 whereinthe device is embedded in a structure and the electrochemical celldetects a parameter indicative of the health of the structure.
 54. Adevice for monitoring the health of a bridge comprising concrete and ametal, the device comprising: a sensor, at least partially exposed tothe concrete, that detects chloride ion presence in the concrete; atransponder in electrical communication with the sensor and thattransmits a wireless signal through the concrete indicating the chlorideion level when triggered by a wireless interrogation signal; anidentification source in electrical communication with the transponderthat uniquely identifies the device; and wherein the device is passive.55. The device of claim 54 wherein the sensor comprises a passiveelectrochemical cell.
 56. The device of claim 54 wherein the transpondercomprises: a ferrite; and a wire wound around the ferrite.
 57. Thedevice of claim 54 wherein the device includes a microchip thatrectifies a signal provided to the transponder.
 58. The device of claim54 further comprising an encapsulation layer comprising concrete thatencapsulates at least the transponder and the identification source. 59.A system for reporting the health of a bridge comprising concrete and ametal, the system comprising: (a) an array of devices, each deviceembedded in the bridge and having: a sensor that detects a parameterindicative of the health of the bridge, a transponder in electricalcommunication with the sensor and that transmits a wireless signalthrough a portion of the concrete indicating the parameter status whentriggered by a wireless interrogation signal, an identification sourcein electrical communication with the transponder that uniquelyidentifies each device from the other devices, and (b) an interrogatorfor externally probing a device in the array to determine the parameterstatus, wherein the interrogator is designed or configured to read theparameter status by (i) providing the wireless interrogation signal tothe transponder and (ii) receiving a wireless response from the device.60. The device of claim 59 wherein each device does not contain a powersource.
 61. The system of claim 60 wherein the interrogator powers thedevice using the wireless interrogation signal.
 62. The system of claim61 wherein the device includes a microchip that rectifies a signalprovided to the transponder.
 63. The device of claim 59 wherein thesensor detects chloride ion concentration in the concrete.
 64. Thesystem of claim 59 wherein interrogator is in digital communication witha database that includes device identifications for each of the devicesin the array.
 65. The system of claim 64 wherein database furthercomprises records identifying the locations of each device in the array.66. The device of claim 59 wherein the sensor detects a level or stateof a chemical species that is aggressive to the health of the structure.67. The device of claim 66 wherein the chemical species is aggressive tothe metal.
 68. The device of claim 67 wherein the threshold correspondsto a level of chloride in the concrete.
 69. The method of claim 59wherein the interrogator is carried by a moving vehicle.
 70. The deviceof claim 59 wherein the sensor detects a level of humidity in theconcrete.
 71. The device of claim 59 wherein the sensor detects thelevel of humidity in the concrete during curing of the concrete.
 72. Amethod for monitoring the health of a structure comprising concrete anda metal, the method comprising: embedding a sensor device in theconcrete, the sensor device comprising a sensor that detects a parameterindicative of the health of the structure, an identification source thatcan distinguish the device from the other similar devices, and atransponder; detecting a level of the parameter using the sensor;probing the device with an interrogator that produces a wireless signalthat transmits through a portion of the concrete; and returning awireless signal from the device through a portion of the concrete,wherein the return wireless signal indicates the parameter status. 73.The method of claim 72 wherein the structure is a bridge or a portion ofa bridge.
 74. The method of claim 73 wherein the structure is a bridgedeck.
 75. The method of claim 72 wherein the wireless signal alsoindicates an identification for the device.
 76. The method of claim 72wherein the interrogator is carried by a moving vehicle while probingthe device.
 77. The method of claim 72 wherein the interrogator iscarried by a person while probing the device.
 78. The method of claim 72further comprising saving the parameter status for the device in memory.79. The method of claim 78 further comprising saving the parameterstatus for the device in a database.
 80. The method of claim 72 furthercomprising compensating the sensed parameter status according to thetemperature of the device.
 81. The method of claim 72 wherein probingthe device comprises powering the device.
 82. The method of claim 72wherein probing the device with the interrogator comprises transmittinga wireless probe signal to the device and receiving a wireless devicesignal from the device.
 83. The method of claim 72 wherein the device isembedded before the concrete finishes curing.